This commit addresses an inefficiency in how enums are created in
pybind11. Most of the enum_<> implementation is completely generic --
however, being a template class, it ended up instantiating vast amounts
of essentially identical code in larger projects with many enums.

This commit introduces a generic non-templated helper class that is
compatible with any kind of enumeration. enum_ then becomes a thin
wrapper around this new class.

The new enum_<> API is designed to be 100% compatible with the old one.

object_api::operator[] has a powerful overload for py::handle that can
accept slices, tuples (for NumPy), etc.

Lists, sequences, and tuples provide their own specialized operator[],
which unfortunately disables this functionality. This is accidental, and
the purpose of this commit is to re-enable the more general behavior.

This commit is tangentially related to the previous one in that it makes
py::handle/py::object et al. behave more like their Python counterparts.

This commit revamps the object_api class so that it maps most C++
operators to their Python analogs. This makes it possible to, e.g.
perform arithmetic using a py::int_ or py::array.

* check for already existing enum value added; added test

* added enum value name to exception message

* test for defining enum with multiple identical names moved to test_enum.cpp/py

* VS 15.8.0 Preview 4.0 has a bug with alias templates

pybind11 headers passed via the `pybind11_add_module` CMake
function can now be included as `SYSTEM` includes (`-isystem`).

This allows to set stricter (or experimental) warnings in
calling projects that might throw otherwise in headers
a user of pybind11 can not influence.

builds against a python debug library were unreliable and
could lead to symbol errors during linking.

Setting the `Py_DEBUG` define is necessary when linking
against a debug build:

https://stackoverflow.com/questions/39161202/how-to-work-around-missing-pymodule-create2-in-amd64-win-python35-d-lib

https://docs.python.org/2/c-api/intro.html#debugging-builds

https://docs.python.org/3.6/c-api/intro.html#debugging-builds

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)];

Catch v2 changed the `run(...)` signature to take a `char *argv[]`,
arguing partly that technically a `char *argv[]` type is the correct
`main()` signature rather than `const char *argv[]`.

Dropping the `const` here doesn't appear to cause any problems with
catch v1 (tested against both the cmake-downloaded 1.9.3 and Debian's
1.12.1 package) so we can follow suit.

* stl.h: propagate return value policies to type-specific casters

Return value policies for containers like those handled in in 'stl.h'
are currently broken.

The problem is that detail::return_value_policy_override<C>::policy()
always returns 'move' when given a non-pointer/reference type, e.g.
'std::vector<...>'.

This is sensible behavior for custom types that are exposed via
'py::class_<>', but it does not make sense for types that are handled by
other type casters (STL containers, Eigen matrices, etc.).

This commit changes the behavior so that
detail::return_value_policy_override only becomes active when the type
caster derives from type_caster_generic.

Furthermore, the override logic is called recursively in STL type
casters to enable key/value-specific behavior.

Switching deprecated Thread Local Storage (TLS) usage in Python 3.7 to Thread Specific Storage (TSS) (#1454)

* Switching deprecated Thread Local Storage (TLS) usage in Python 3.7 to Thread Specific Storage (TSS)

* Changing Python version from 3.6 to 3.7 for Travis CI, to match brew's version of Python 3

* Introducing PYBIND11_ macros to switch between TLS and TSS API

The current code requires implicitly that integral types are cast-able to floating point. In case of strongly-typed integrals (e.g. as explained at http://www.ilikebigbits.com/blog/2014/5/6/type-safe-identifiers-in-c) this is not always the case.

This commit uses SFINAE to move the numeric conversions into separate `cast()` implementations to avoid the issue.

* Silence GCC8's -Wcast-function-type.

See https://bugs.python.org/issue33012 and PRs linked therein.

It is useful not only to remember the python libs and includes but
also the interpreter version in cache.

If users call pybind11 throught `add_subdirectories` they will
otherwise have no access to the selected interpreter version.
The interpreter version is useful for downstream projects, e.g.
to select default `lib/pythonX.Y/site-packages/` install paths.

Add missing python3 distutils on Debian "buster".

If an exception is thrown during module initialization, the
error_already_set destructor will try to call `get_internals()` *after*
setting Python's error indicator, resulting in a `SystemError: ...
returned with an error set`.

Fix that by temporarily stashing away the error indicator in the
destructor.

Fixes #1401

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

Fix a segfault when creating a 0-dimension, c-strides array.

Found via `codespell`

When using pybind11 to bind enums on MSVC and warnings (/W4) enabled,
the following warning pollutes builds. This fix renames one of the
occurrences.

pybind11\include\pybind11\pybind11.h(1398): warning C4459: declaration of 'self' hides global declaration
pybind11\include\pybind11\operators.h(41): note: see declaration of 'pybind11::detail::self'