[skip ci]

Fixes #887.

`accessor` currently relies on an implicit default copy constructor, but that is deprecated in C++11 when a copy assignment operator is present and can, in some cases, raise deprecation warnings (see #888).  This commit explicitly specifies the default copy constructor and also adds a default move constructor.

[skip ci]

This reimplements the std::reference_wrapper<T> caster to be a shell
around the underlying T caster (rather than assuming T is a generic
type), which lets it work for things like `std::reference_wrapper<int>`
or anything else custom type caster with a lvalue cast operator.

This also makes it properly fail when None is provided, just as an
ordinary lvalue reference argument would similarly fail.

This also adds a static assert to test that T has an appropriate type
caster.  It triggers for casters like `std::pair`, which have
return-by-value cast operators.  (In theory this could be supported by
storing a local temporary for such types, but that's beyond the scope
of this PR).

This also replaces `automatic` or `take_ownership` return value policies
with `automatic_reference` as taking ownership of a reference inside a
reference_wrapper is not valid.

test_issues is deprecated, and the following commit adds other, related
reference_wrapper tests.

The new version of pytest now reports Python warnings by default. This
commit filters out some third-party extension warnings which are not
useful for pybind11 tests.

This commit also adds `doc()` to `object_api` as a shortcut for the
`attr("__doc__")` accessor.

The module macro changes from:
```c++
PYBIND11_PLUGIN(example) {
    pybind11::module m("example", "pybind11 example plugin");
    m.def("add", [](int a, int b) { return a + b; });
    return m.ptr();
}
```

to:

```c++
PYBIND11_MODULE(example, m) {
    m.doc() = "pybind11 example plugin";
    m.def("add", [](int a, int b) { return a + b; });
}
```

Using the old macro results in a deprecation warning. The warning
actually points to the `pybind11_init` function (since attributes
don't bind to macros), but the message should be quite clear:
"PYBIND11_PLUGIN is deprecated, use PYBIND11_MODULE".

* Added template constructors to buffer_info that can deduce the item size, format string, and number of dimensions from the pointer type and the shape container

* Implemented actual buffer_info constructor as private delegate constructor taking rvalue reference as a workaround for the evaluation order move problem on GCC 4.8

Fixes #878.

At this point, there is only a single test for interpreter basics.

Apart from embedding itself, having a C++ test framework will also
benefit the C++-side features by allowing them to be tested directly.

All targets provided by pybind11:

* pybind11::module - the existing target for creating extension modules
* pybind11::embed - new target for embedding the interpreter
* pybind11::pybind11 - common "base" target (headers only)

Now that #851 has removed all multiple uses of a caster, it can just use
the default-constructed value with needing a reset. This fixes two
issues:

1. With std::experimental::optional (at least under GCC 5.4), the `= {}`
would construct an instance of the optional type and then move-assign
it, which fails if the value type isn't move-assignable.

2. With older versions of Boost, the `= {}` could fail because it is
ambiguous, allowing construction of either `boost::none` or the value
type.

:exc: isn't valid.

The stl_bind.h wrapper for `Vector.insert` neglected to do a bounds
check.

The "extend" method for vectors defined in stl_bind.h used `reserve` to
allocate space for the extra growth. While this can sometimes make a
constant-factor improvement in performance, it can also cause
construction of a vector by repeated extension to take quadratic rather
than linear time, as memory is reallocated in small increments rather
than on an exponential schedule. For example, this Python code would
take time proportional to the square of the trip count:

```python
a = VectorInt([1, 2, 3])
b = VectorInt()
for i in range(100000):
    b.extend(a)
```

This commit removes the `reserve` call. The alternative would be to try
to add some smarter heuristics, but the standard library may well have
its own heuristics (the iterators are random access iterators, so it can
easily determine the number of items being added) and trying to add more
heuristics on top of that seems like a bad idea.

MSVC by default uses the local codepage, which fails when it sees the
utf-8 in test_python_types.cpp.  This adds the /utf-8 flag to the test
suite compilation to force it to interpret source code as utf-8.

Fixes #869

This extends py::vectorize to automatically pass through
non-vectorizable arguments.  This removes the need for the documented
"explicitly exclude an argument" workaround.

Vectorization now applies to arithmetic, std::complex, and POD types,
passed as plain value or by const lvalue reference (previously only
pass-by-value types were supported).  Non-const lvalue references and
any other types are passed through as-is.

Functions with rvalue reference arguments (whether vectorizable or not)
are explicitly prohibited: an rvalue reference is inherently not
something that can be passed multiple times and is thus unsuitable to
being in a vectorized function.

The vectorize returned value is also now more sensitive to inputs:
previously it would return by value when all inputs are of size 1; this
is now amended to having all inputs of size 1 *and* 0 dimensions.  Thus
if you pass in, for example, [[1]], you get back a 1x1, 2D array, while
previously you got back just the resulting single value.

Vectorization of member function specializations is now also supported
via `py::vectorize(&Class::method)`; this required passthrough support
for the initial object pointer on the wrapping function pointer.

Currently if you construct an `array_t<T, array::f_style>` with a shape
but not strides you get a C-style array; the only way to get F-style
strides was to calculate the strides manually.  This commit fixes that
by adding logic to use f_style strides when the flag is set.

This also simplifies the existing c_style stride logic.

Adds `remove_reference_t` and converts various `typename
std::remove_reference<...>::type` to using it.

This allows calling of functions (typically void) over a parameter
pack, replacing usage such as:

    bool unused[] = { (voidfunc(param_pack_arg), false)..., false };
    (void) unused;

with a much cleaner:

    PYBIND11_EXPAND_SIDE_EFFECTS(voidfunc(param_pack_arg));

This attribute lets you disable (or explicitly enable) passing None to
an argument that otherwise would allow it by accepting
a value by raw pointer or shared_ptr.

This commit allows type_casters to allow their local values to be moved
away, rather than copied, when the type caster instance itself is an rvalue.

This only applies (automatically) to type casters using
PYBIND11_TYPE_CASTER; the generic type type casters don't own their own
pointer, and various value casters (e.g. std::string, std::pair,
arithmetic types) already cast to an rvalue (i.e. they return by value).

This updates various calling code to attempt to get a movable value
whenever the value is itself coming from a type caster about to be
destroyed: for example, when constructing an std::pair or various stl.h
containers.  For types that don't support value moving, the cast_op
falls back to an lvalue cast.

There wasn't an obvious place to add the tests, so I added them to
test_copy_move_policies, but also renamed it to drop the _policies as it
now tests more than just policies.

Closes #857, by adding overloads to def_buffer that match pointers to
member functions and wrap them in lambdas.

This changes javadoc-style documenting comments from:

    /** Text starts here
     * and continues here
     */

to:

    /**
     * Test starts here
     * and continues here
     */

which looks a little better, and also matches the javadoc-recommended
way of writing documenting comments.

Using a dynamic_cast instead of a static_cast is needed to safely cast
from a base to a derived type.  The previous static_pointer_cast isn't
safe, however, when downcasting (and fails to compile when downcasting
with virtual inheritance).

Switching this to always use a dynamic_pointer_cast shouldn't incur any
additional overhead when a static_pointer_cast is safe (i.e. when
upcasting, or self-casting): compilers don't need RTTI checks in those
cases.

Python 2 requires both `__div__` and `__truediv__` (and variants) for
compatibility with both regular Python 2 and Python 2 under `from
__future__ import division`.  Without both, division fails in one or the
other case.

- realign \ at end of macro lines
- use 'using A = B;' rather than 'typedef B A;'
- use conditional_t

The Python method for /= was set as `__idiv__`, which should be
`__itruediv__` under Python 3.

This wasn't totally broken in that without it defined, Python constructs
a new object by calling __truediv__.  The operator tests, however,
didn't actually test the /= operator: when I added it, I saw an extra
construction, leading to the problem.  This commit also includes tests
for the previously untested *= operator, and adds some element-wise
vector multiplication and division operators.

The numpy strides/sizes/etc. are signed now, but the static_assert
didn't get updated to match.

Under gcc, the `static internals *internals_ptr` is shared across .so's,
which breaks for obvious reasons.

This commit fixes it by moving the static pointer declaration into a
pybind-version-templated function.

Fixes #852.

Missing conformability check was causing Eigen to create a 0x0 matrix
with an error in debug mode and silent corruption in release mode.

Currently, `py::int_(1).cast<variant<double, int>>()` fills the `double`
slot of the variant. This commit switches the loader to a 2-pass scheme
in order to correctly fill the `int` slot.

Many of our `is_none()` checks in type caster loading return true, but
this should really be considered a deferral so that, for example, an
overload with a `py::none` argument would win over one that takes
`py::none` as a null option.

This keeps None-accepting for the `!convert` pass only for std::optional
and void casters.  (The `char` caster already deferred None; this just
extends that behaviour to other casters).

Under gcc 7 with -std=c++11, compilation results in several of the
following warnings:

    In file included from /home/jagerman/src/pybind11/tests/test_sequences_and_iterators.cpp:13:0:
    /home/jagerman/src/pybind11/include/pybind11/operators.h: In function ‘pybind11::detail::op_<(pybind11::detail::op_id)0, (pybind11::detail::op_type)0, pybind11::detail::self_t, pybind11::detail::self_t> pybind11::detail::operator+(const pybind11::detail::self_t&, const pybind11::detail::self_t&)’:
    /home/jagerman/src/pybind11/include/pybind11/operators.h:78:76: warning: inline declaration of ‘pybind11::detail::op_<(pybind11::detail::op_id)0, (pybind11::detail::op_type)0, pybind11::detail::self_t, pybind11::detail::self_t> pybind11::detail::operator+(const pybind11::detail::self_t&, const pybind11::detail::self_t&)’ follows declaration with attribute noinline [-Wattributes]
     inline op_<op_##id, op_l, self_t, self_t> op(const self_t &, const self_t &) {         \
                                                                                ^
    /home/jagerman/src/pybind11/include/pybind11/operators.h:109:1: note: in expansion of macro ‘PYBIND11_BINARY_OPERATOR’
     PYBIND11_BINARY_OPERATOR(add,       radd,         operator+,    l + r)
     ^~~~~~~~~~~~~~~~~~~~~~~~
    In file included from /home/jagerman/src/pybind11/include/pybind11/cast.h:15:0,
                     from /home/jagerman/src/pybind11/include/pybind11/attr.h:13,
                     from /home/jagerman/src/pybind11/include/pybind11/pybind11.h:36,
                     from /home/jagerman/src/pybind11/tests/pybind11_tests.h:2,
                     from /home/jagerman/src/pybind11/tests/test_sequences_and_iterators.cpp:11:
    /home/jagerman/src/pybind11/include/pybind11/descr.h:116:36: note: previous definition of ‘pybind11::detail::descr pybind11::detail::operator+(pybind11::detail::descr&&, pybind11::detail::descr&&)’ was here
         PYBIND11_NOINLINE descr friend operator+(descr &&d1, descr &&d2) {
                                        ^~~~~~~~

This appears to be happening because gcc is considering implicit
construction of `descr` in some places using addition of two
`descr`-compatible arguments in the `descr.h` c++11 fallback code.
There's no particular reason that this operator needs to be a friend
function: this commit changes it to an rvalue-context member function
operator, which avoids the warning.