Creating an instance of of a pybind11-bound type caused a reference leak in the
associated Python type object, which could prevent these from being collected
upon interpreter shutdown. This commit fixes that issue for all types that are
defined in a scope (e.g. a module). Unscoped anonymous types (e.g. custom
iterator types) always retain a positive reference count to prevent their
collection.

For the case of `pip install --user`, the header include dirs must
also include `pybind11.get_include(True)`.

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The default `install_headers` from `distutils` flattens all the headers
into a single directory -- `detail` subdirectory was lost. This commit
fixes this by overriding the setup with a custom header installer.

Tests are added to Travis to make sure `setup.py sdist` and `pip install`
do not miss any headers and that the directory structure is preserved.

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Newer clang produces additional warnings.

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The `latest` build remains as is, but all others are modified to:

* Use regular Python instead of conda. `pip install` is much faster
  than conda, but scipy isn't available. Numpy is still tested.

* Compile in debug mode instead of release.

* Skip CMake build tests. For some reason, CMake configuration is very
  slow on AppVeyor and these tests are almost entirely CMake.

The changes reduce build time to about 1/3 of the original. The `latest` 
config still covers scipy, release mode and the CMake build tests, so 
the others don't need to.

The current PYBIND11_INTERNALS_ID depends on the version of the library
in order to isolate binary incompatible internals capsules. However,
this does not preclude conflicts between modules built from different
(binary incompatible) commits with the same version number.

For example, if one module was built with an early v2.2.dev and
submitted to PyPI, it could not be loaded alongside a v2.2.x release
module -- it would segfault because of incompatible internals with
the same ID.

This PR changes the ID to depend on PYBIND11_INTERNALS_VERSION which is
independent of the main library version. It's an integer which should be
incremented whenever a binary incompatible change is made to internals.

PYBIND11_INTERNALS_KIND is also introduced for a similar reason.

The same versioning scheme is also applied to `type_info` and the
`module_local` type attribute.

The "see above" comment being referenced in the code comments isn't
"above" anymore; copy the later factory init comment into the first
constructor block to fix it.

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[skip ci]

Fixes one small variable name typo, and two instances where `py::arg().nocopy()` is used, where I think it should be `py::arg().noconvert()` instead. Probably `nocopy()` was the old/original name for it and then it was changed.

The main point of `py::module_local` is to make the C++ -> Python cast
unique so that returning/casting a C++ instance is well-defined.
Unfortunately it also makes loading unique, but this isn't particularly
desirable: when an instance contains `Type` instance there's no reason
it shouldn't be possible to pass that instance to a bound function
taking a `Type` parameter, even if that function is in another module.

This commit solves the issue by allowing foreign module (and global)
type loaders have a chance to load the value if the local module loader
fails.  The implementation here does this by storing a module-local
loading function in a capsule in the python type, which we can then call
if the local (and possibly global, if the local type is masking a global
type) version doesn't work.

The macro isn't doing anything useful now that hidden visibility is
applied to all pybind11 code.

This reimplements the py::init<...> implementations using the various
functions added to support `py::init(...)`, and moves the implementing
structs into `detail/init.h` from `pybind11.h`.  It doesn't simply use a
factory directly, as this is a very common case and implementation
without an extra lambda call is a small but useful optimization.

This, combined with the previous lazy initialization, also avoids
needing placement new for `py::init<...>()` construction: such
construction now occurs via an ordinary `new Type(...)`.

A consequence of this is that it also fixes a potential bug when using
multiple inheritance from Python: it was very easy to write classes
that double-initialize an existing instance which had the potential to
leak for non-pod classes.  With the new implementation, an attempt to
call `__init__` on an already-initialized object is now ignored.  (This
was already done in the previous commit for factory constructors).

This change exposed a few warnings (fixed here) from deleting a pointer
to a base class with virtual functions but without a virtual destructor.
These look like legitimate warnings that we shouldn't suppress; this
adds virtual destructors to the appropriate classes.

This allows you to use:

    cls.def(py::init(&factory_function));

where `factory_function` returns a pointer, holder, or value of the
class type (or a derived type).  Various compile-time checks
(static_asserts) are performed to ensure the function is valid, and
various run-time type checks where necessary.

Some other details of this feature:
- The `py::init` name doesn't conflict with the templated no-argument
  `py::init<...>()`, but keeps the naming consistent: the existing
  templated, no-argument one wraps constructors, the no-template,
  function-argument one wraps factory functions.
- If returning a CppClass (whether by value or pointer) when an CppAlias
  is required (i.e. python-side inheritance and a declared alias), a
  dynamic_cast to the alias is attempted (for the pointer version); if
  it fails, or if returned by value, an Alias(Class &&) constructor
  is invoked.  If this constructor doesn't exist, a runtime error occurs.
- for holder returns when an alias is required, we try a dynamic_cast of
  the wrapped pointer to the alias to see if it is already an alias
  instance; if it isn't, we raise an error.
- `py::init(class_factory, alias_factory)` is also available that takes
  two factories: the first is called when an alias is not needed, the
  second when it is.
- Reimplement factory instance clearing.  The previous implementation
  failed under python-side multiple inheritance: *each* inherited
  type's factory init would clear the instance instead of only setting
  its own type value.  The new implementation here clears just the
  relevant value pointer.
- dealloc is updated to explicitly set the leftover value pointer to
  nullptr and the `holder_constructed` flag to false so that it can be
  used to clear preallocated value without needing to rebuild the
  instance internals data.
- Added various tests to test out new allocation/deallocation code.
- With preallocation now done lazily, init factory holders can
  completely avoid the extra overhead of needing an extra
  allocation/deallocation.
- Updated documentation to make factory constructors the default
  advanced constructor style.
- If an `__init__` is called a second time, we have two choices: we can
  throw away the first instance, replacing it with the second; or we can
  ignore the second call.  The latter is slightly easier, so do that.

An alias can be used for two main purposes: to override virtual methods,
and to add some extra data to a class needed for the pybind-wrapper.
Both of these absolutely require that the wrapped class be polymorphic
so that virtual dispatch and destruction, respectively, works.

`function_signature_t` extracts the function type from a function,
function pointer, or lambda.

`is_lambda` (which is really
`is_not_a_function_or_pointer_or_member_pointer`, but that name is a
bit too long) checks whether the type is (in the approprate context) a
lambda.

`is_function_pointer` checks whether the type is a pointer to a
function.

We currently allocate instance values when creating the instance itself
(except when constructing the instance for a `cast()`), but there is no
particular reason to do so: the instance itself and the internals (for
a non-simple layout) are allocated via Python, with no reason to
expect better locality from the invoked `operator new`.  Moreover, it
makes implementation of factory function constructors trickier and
slightly less efficient: they don't use the pre-eallocate the memory,
which means there is a pointless allocation and free.

This commit makes the allocation lazy: instead of preallocating when
creating the instance, the allocation happens when the instance is
first loaded (if null at that time).

In addition to making it more efficient to deal with cases that don't
need preallocation, this also allows for a very slight performance
increase by not needing to look up the instances types during
allocation.  (There is a lookup during the eventual load, of course, but
that is happening already).

* Doxygen needs `RECURSIVE = YES` in order to parse the `detail` subdir.

* The `-W` warnings-as-errors option for sphinx doesn't work with the
  makefile build. Switched to calling sphinx directly.

* Fix "citation [cppimport] is not referenced" warning.

Embedding may well be used in places where hidden visibility isn't
desired.  It should be relatively safe to allow it there; any potential
conflict would come in if modules are loaded into that embedded
interpreter, but as long as the modules are compiled with hidden
visibility they shouldn't conflict.

There could still be warnings if the embedded code attempts to export
classes with internal (hidden) pybind members, but that seems a
legitimate warning (and already has a FAQ entry).

CMAKE_CXX_STANDARD is only available on CMake >= 3.1. If the flag is
set, we avoid initializing PYBIND11_CPP_STANDARD.

This updates the compilation to always apply hidden visibility to
resolve the issues with default visibility causing problems under debug
compilations.  Moreover using the cmake property makes it easier for a
caller to override if absolutely needed for some reason.

For `pybind11_add_module` we use cmake to set the property; for the
targets, we append to compilation option to non-MSVC compilers.

This adds a PYBIND11_NAMESPACE macro that expands to the `pybind11`
namespace with hidden visibility under gcc-type compilers, and otherwise
to the plain `pybind11`.  This then forces hidden visibility on
everything in pybind, solving the visibility issues discussed at end
end of #949.

One module uses a generic vector caster from `<pybind11/stl.h>` while
the other exports `std::vector<int>` with a local `py:bind_vector`.

This should mitigate accidental invocation on a temporary array.

Fixes #961.

In C++11 mode, `boost::apply_visitor` requires an explicit `result_type`.
This also adds optional tests for `boost::variant` in C++11/14, if boost
is available. In C++17 mode, `std::variant` is tested instead.

* Ensure :ref: for virtual_and_inheritance is parsed.

* Add quick blurb about __init__ with inherited types.

[skip ci]

When Pybind11 is used via `add_subdirectory`, when targets are installed
from the parent project, CMake wants all of the dependencies built by
the project in the same export set. Projects may now set
`PYBIND11_EXPORT_NAME` to have Pybind11 put it targets into the
project's export set. If so, do not install Pybind11's export file.