* Use LIBDIR and MULTIARCH on linux to find python library

* Remove apple-specific setting; the non-windows one should work fine on OS X

* Default LIBDIR/MULTIARCH to '' (to avoid getting None)

* Remove trailing whitespace from FindPythonLibsNew

Previously all types are marked unaligned in buffer format strings,
now we test for alignment before adding the '=' marker.

This gives more informative output, often including the type (or at
least some hint about the type).

A flake8 configuration is included in setup.cfg and the checks are
executed automatically on Travis:

* Ensures a consistent PEP8 code style
* Does basic linting to prevent possible bugs

Fixes #509.

The move policy was already set for rvalues in PR #473, but this only
applied to directly cast user-defined types. The problem is that STL
containers cast values indirectly and the rvalue information is lost.
Therefore the move policy was not set correctly. This commit fixes it.

This also makes an additional adjustment to remove the `copy` policy
exception: rvalues now always use the `move` policy. This is also safe
for copy-only rvalues because the `move` policy has an internal fallback
to copying.

Following commit 90d278, the object code generated by the python
bindings of nanogui (github.com/wjakob/nanogui) went up by a whopping
12%. It turns out that that project has quite a few enums where we don't
really care about arithmetic operators.

This commit thus partially reverts the effects of #503 by introducing
an additional attribute py::arithmetic() that must be specified if the
arithmetic operators are desired.

* `array_t(const object &)` now throws on error
* `array_t::ensure()` is intended for casters —- old constructor is
  deprecated
* `array` and `array_t` get default constructors (empty array)
* `array` gets a converting constructor
* `py::isinstance<array_T<T>>()` checks the type (but not flags)

There is only one special thing which must remain: `array_t` gets
its own `type_caster` specialization which uses `ensure` instead
of a simple check.

The pytype converting constructors are convenient and safe for user
code, but for library internals the additional type checks and possible
conversions are sometimes not desired. `reinterpret_borrow<T>()` and
`reinterpret_steal<T>()` serve as the low-level unsafe counterparts
of `cast<T>()`.

This deprecates the `object(handle, bool)` constructor.

Renamed `borrowed` parameter to `is_borrowed` to avoid shadowing
warnings on MSVC.

* Deprecate the `py::object::str()` member function since `py::str(obj)`
  is now equivalent and preferred

* Make `py::repr()` a free function

* Make sure obj.cast<T>() works as expected when T is a Python type

`obj.cast<T>()` should be the same as `T(obj)`, i.e. it should convert
the given object to a different Python type. However, `obj.cast<T>()`
usually calls `type_caster::load()` which only checks the type without
doing any actual conversion. That causes a very unexpected `cast_error`.
This commit makes it so that `obj.cast<T>()` and `T(obj)` are the same
when T is a Python type.

* Simplify pytypes converting constructor implementation

It's not necessary to maintain a full set of converting constructors
and assignment operators + const& and &&. A single converting const&
constructor will work and there is no impact on binary size. On the
other hand, the conversion functions can be significantly simplified.

Allows checking the Python types before creating an object instead of
after. For example:
```c++
auto l = list(ptr, true);
if (l.check())
   // ...
```
The above is replaced with:
```c++
if (isinstance<list>(ptr)) {
    auto l = reinterpret_borrow(ptr);
    // ...
}
```

This deprecates `py::object::check()`. `py::isinstance()` covers the
same use case, but it can also check for user-defined types:
```c++
class Pet { ... };
py::class_<Pet>(...);

m.def("is_pet", [](py::object obj) {
    return py::isinstance<Pet>(obj); // works as expected
});
```

* Also added unsafe version without checks

* Allow enums to be ordered
* Support binary operators

* Incref returned None in std::optional type caster

* Add type casters for nullopt_t

* Add a test for nullopt_t

Added the docstring_options class, which gives global control over the generation of docstrings and function signatures.

This commit includes the following changes:

* Don't provide make_copy_constructor for non-copyable container

make_copy_constructor currently fails for various stl containers (e.g.
std::vector, std::unordered_map, std::deque, etc.) when the container's
value type (e.g. the "T" or the std::pair<K,T> for a map) is
non-copyable.  This adds an override that, for types that look like
containers, also requires that the value_type be copyable.

* stl_bind.h: make bind_{vector,map} work for non-copy-constructible types

Most stl_bind modifiers require copying, so if the type isn't copy
constructible, we provide a read-only interface instead.

In practice, this means that if the type is non-copyable, it will be,
for all intents and purposes, read-only from the Python side (but
currently it simply fails to compile with such a container).

It is still possible for the caller to provide an interface manually
(by defining methods on the returned class_ object), but this isn't
something stl_bind can handle because the C++ code to construct values
is going to be highly dependent on the container value_type.

* stl_bind: copy only for arithmetic value types

For non-primitive types, we may well be copying some complex type, when
returning by reference is more appropriate.  This commit returns by
internal reference for all but basic arithmetic types.

* Return by reference whenever possible

Only if we definitely can't--i.e. std::vector<bool>--because v[i]
returns something that isn't a T& do we copy; for everything else, we
return by reference.

For the map case, we can always return by reference (at least for the
default stl map/unordered_map).

When working on some particular feature, it's nice to be able to disable
all the tests except for the one I'm working on; this is currently
possible by editing tests/CMakeLists.txt, and commenting out the tests
you don't want.

This commit goes a step further by letting you give a list of tests you
do want when invoking cmake, e.g.:

    cmake -DPYBIND11_TEST_OVERRIDE="test_issues.cpp;test_pickling.cpp" ..

changes the build to build just those two tests (and changes the `pytest`
target to invoke just the two associated tests).

This persists in the build directory until you disable it again by
running cmake with `-DPYBIND11_TEST_OVERRIDE=`.  It also adds a message
after the pytest output to remind you that it is in effect:

    Note: not all tests run: -DPYBIND11_TEST_OVERRIDE is in effect

If we need to initialize a holder around an unowned instance, and the
holder type is non-copyable (i.e. a unique_ptr), we currently construct
the holder type around the value pointer, but then never actually
destruct the holder: the holder destructor is called only for the
instance that actually has `inst->owned = true` set.

This seems no pointer, however, in creating such a holder around an
unowned instance: we never actually intend to use anything that the
unique_ptr gives us: and, in fact, do not want the unique_ptr (because
if it ever actually got destroyed, it would cause destruction of the
wrapped pointer, despite the fact that that wrapped pointer isn't
owned).

This commit changes the logic to only create a unique_ptr holder if we
actually own the instance, and to destruct via the constructed holder
whenever we have a constructed holder--which will now only be the case
for owned-unique-holder or shared-holder types.

Other changes include:

* Added test for non-movable holder constructor/destructor counts

The three alive assertions now pass, before #478 they fail with counts
of 2/2/1 respectively, because of the unique_ptr that we don't want and
don't destroy (because we don't *want* its destructor to run).

* Return cstats reference; fix ConstructStats doc

Small cleanup to the #478 test code, and fix to the ConstructStats
documentation (the static method definition should use `reference` not
`reference_internal`).

* Rename inst->constructed to inst->holder_constructed

This makes it clearer exactly what it's referring to.

There are now more places than just descr.h that make use of these.
The new macro isn't quite the same: the old one only tested for a
couple features, while the new one checks for the __cplusplus version
(but doesn't even try to enable C++14 for MSVC/ICC).

g++ 7 adds <optional>, but including it in C++14 mode isn't allowed
(just as including <experimental/optional> isn't allowed in C++11 mode).
(This wasn't triggered in g++-6 because it doesn't provide <optional>
yet.)

* Add debugging info about so size to build output

This adds a small python script to tools that captures before-and-after
.so sizes between builds and outputs this in the build output via a
string such as:

------ pybind11_tests.cpython-35m-x86_64-linux-gnu.so file size: 924696 (decrease of 73680 bytes = 7.38%)

------ pybind11_tests.cpython-35m-x86_64-linux-gnu.so file size: 998376 (increase of 73680 bytes = 7.97%)

------ pybind11_tests.cpython-35m-x86_64-linux-gnu.so file size: 998376 (no change)

Or, if there was no .so during the build, just the .so size by itself:

------ pybind11_tests.cpython-35m-x86_64-linux-gnu.so file size: 998376

This allows you to, for example, build, checkout a different branch,
rebuild, and easily see exactly the change in the pybind11_tests.so
size.

It also allows looking at the travis and appveyor build logs to get an
idea of .so/.dll sizes across different build systems.

* Minor libsize.py script changes

- Use RAII open
- Remove unused libsize=-1
- Report change as [+-]xyz bytes = [+-]a.bc%