Currently user specializations of the form

template <typename itype> struct polymorphic_type_hook<itype, std::enable_if_t<...>> { ... };

will fail if itype is also polymorphic, because the existing specialization will also
be enabled, which leads to 2 equally viable candidates. With this change, user provided
specializations have higher priority than the built in specialization for polymorphic types.

When binding code immediately throws an exception of type
py::error_already_set (e.g. via py::module::import that fails), the
catch block sets an import error as expected. Unfortunately, following
this, the deconstructor of py::error_already_set decides to call
py::detail::get_internals() and set up various internal data structures
of pybind11, which fails given that the error flag is active. The call
stack of this looks as follows:

Py_init_mymodule() -> __cxa_decrement_exception_refcount ->
error_already_set::~error_already_set() ->
gil_scoped_acquire::gil_scoped_acquire() -> detail::get_internals() ->
... -> pybind11::detail::simple_collector() -> uh oh..

The solution is simple: we call detail::get_internals() once before
running any binding code to make sure that the internal data structures
are ready.

This commit introduces the use of C++17-style fold expressions when
casting tuples & the argument lists of functions.

This change can improve performance of the resulting bindings: because
fold expressions have short-circuiting semantics, pybind11 e.g. won't
try to cast the second argument of a function if the first one failed.
This is particularly effective when working with functions that have
many overloads with long argument lists.

* Fix test build in C++20

* Add C++20 char8_t/u8string support

If a debug interpreter is detected, we allow linking with
pythonXX_d.lib under windows.

This reverts commit 819802da.

This reverts commit 61e4f118.

* Add FAQ entry, with code example, for dealing with long functions interruption

Fixing SystemError when nb_bool/nb_nonzero sets a Python exception in type_caster<bool>::load (#1976)

Add Python 3.8 to considered versions in CMake for additional
hints.
https://cmake.org/cmake/help/v3.2/module/FindPythonLibs.html

* test pair-copyability on C++17 upwards

The stdlib falsely detects containers like M=std::map<T, U>
as copyable, even when one of T and U is not copyable.
Therefore we cannot rely on the stdlib dismissing std::pair<T, M>
by itself, even on C++17.

* fix is_copy_assignable

bind_map used std::is_copy_assignable which suffers from the same problems
as std::is_copy_constructible, therefore the same fix has been applied.

* created tests for copyability

Don't assume that just because the language version is C++17 that the
standard library offers all C++17 features, too.  When using clang-6.0
and --std=c++17 on Ubuntu 18.04 with libstdc++, __cpp_sized_deallocation
is false.

When building with `-Werror,-Wmissing-prototypes`, `clang` complains about missing prototypes for functions defined through macro expansions. This PR adds the missing prototypes.

```
error: no previous prototype for function 'pybind11_init_impl_embedded' [
-Werror,-Wmissing-prototypes]                                           
PYBIND11_EMBEDDED_MODULE(embedded, mod) {                                             
^                                                                           
external/pybind11/include/pybind11/embed.h:61:5: note: expanded from macro 'PYBIND11_EMBEDDED_MODULE'
    PYBIND11_EMBEDDED_MODULE_IMPL(name)                                       \
    ^                                                                         
external/pybind11/include/pybind11/embed.h:26:23: note: expanded from macro 'PYBIND11_EMBEDDED_MODULE_IMPL'
      extern "C" void pybind11_init_impl_##name() {      \                  
                      ^                                             
<scratch space>:380:1: note: expanded from here                     
pybind11_init_impl_embedded                                                                                                                      
^                                                                                                                                                
1 error generated.
```