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* doc: Add note about casting from `None` to `T*`

* doc: reword 'none-to-pointer' note message

* doc: mention opaque types in 'none-to-pointer' note message

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* Adds std::deque to the types supported by list_caster in stl.h.
* Adds a new test_deque test in test_stl.{py,cpp}.
* Updates the documentation to include std::deque as a default
  supported type.

This PR adds a new py::ellipsis() method which can be used in
conjunction with NumPy's generalized slicing support. For instance,
the following is now valid (where "a" is a NumPy array):

py::array b = a[py::make_tuple(0, py::ellipsis(), 0)];

Fixed typos (erroneous `;`) in `classes.rst`.

Found via `codespell`

* Add basic support for tag-based static polymorphism

Sometimes it is possible to look at a C++ object and know what its dynamic type is,
even if it doesn't use C++ polymorphism, because instances of the object and its
subclasses conform to some other mechanism for being self-describing; for example,
perhaps there's an enumerated "tag" or "kind" member in the base class that's always
set to an indication of the correct type. This might be done for performance reasons,
or to permit most-derived types to be trivially copyable. One of the most widely-known
examples is in LLVM: https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html

This PR permits pybind11 to be informed of such conventions via a new specializable
detail::polymorphic_type_hook<> template, which generalizes the previous logic for
determining the runtime type of an object based on C++ RTTI. Implementors provide
a way to map from a base class object to a const std::type_info*...

I think that there's the word "for" missing for that sentence to be correct.
Please double-check that the sentence means what it's supposed to mean. :-)

- PYBIND11_MAKE_OPAQUE now takes ... rather than a single argument and
  expands it with __VA_ARGS__; this lets templated, comma-containing
  types get through correctly.
- Adds a new macro PYBIND11_TYPE() that lets you pass the type into a
  macro as a single argument, such as:

      PYBIND11_OVERLOAD(PYBIND11_TYPE(R<1,2>), PYBIND11_TYPE(C<3,4>), func)

  Unfortunately this only works for one macro call: to forward the
  argument on to the next macro call (without the processor breaking it
  up again) requires also adding the PYBIND11_TYPE(...) to type macro
  arguments in the PYBIND11_OVERLOAD_... macro chain.
- updated the documentation with these two changes, and use them at a couple
  places in the test suite to test that they work.

Found via `codespell`

None of the three currently recommended approaches works on PyPy, due to
it not garbage collecting things when you want it to. Added a note with
example showing how to get interpreter shutdown callbacks using the Python
atexit module.

Non-user facing. 
Found using `codespell -q 3`

This also matches the Eigen example for the row-major case.

This also enhances one of the tests to trigger a failure (and fixes it in the PR).  (This isn't really a flaw in pybind itself, but rather fixes wrong code in the test code and docs).

* Expand documentation to include explicit example of py::module::import 
  where one would expect it.

* Describe how to use unbound and bound methods to class Python classes.

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PR #880 changed the implementation of keep_alive to avoid weak
references when the nurse is pybind11-registered, but the documentation
didn't get updated to match.

This is analogous to `py::init()` vs `__init__` + placement-new.
`py::pickle()` reuses most of the implementation details of `py::init()`.

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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.

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.

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.

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.

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boost::apply_visitor accepts its arguments by non-const lvalue
reference, which fails to bind to an rvalue reference.  Change the
example to remove the argument forwarding.