bpo-35134, bpo-40421: Add Include/cpython/code.h header file.

code.h now defines PyCodeObject type in the limited C API. It is now
included by Python.h.

Give a name to the PyCodeObject structure: it is now called
"struct PyCodeObject". So it becomes possible to define PyCodeObject
as "struct PyCodeObject" in the limited C API without defining the
structure.

Add a new separated pyframe.h header file of the PyFrame public C
API: it is included by Python.h.

Add PyFrame_GetLineNumber() to the limited C API.

Replace "struct _frame" with "PyFrameObject" in header files.
PyFrameObject is now defined as struct _frame by pyframe.h which is
included early enough in Python.h.

The changes in this commit are all related to @vstinner's original review comments of the initial PEP 617 implementation PR.

Co-authored-by: Guido van Rossum <guido@python.org>
Co-authored-by: Lysandros Nikolaou <lisandrosnik@gmail.com>

Add a new internal pycore_byteswap.h header file with the following
functions:

* _Py_bswap16()
* _Py_bswap32()
* _Py_bswap64()

Use these functions in _ctypes, sha256 and sha512 modules,
and also use in the UTF-32 encoder.

sha256, sha512 and _ctypes modules are now built with the internal
C API.

Move PyInterpreterState and related functions to a new internal
pycore_interp.h header file.

Move PyRuntimeState from pycore_pystate.h to pycore_runtime.h.

Remove _PyGILState_check_enabled macro: access directly
_PyRuntime.gilstate.check_enabled.

Add _PyIndex_Check() function to the internal C API: fast inlined
verson of PyIndex_Check().

Add Include/internal/pycore_abstract.h header file.

Replace PyIndex_Check() with _PyIndex_Check() in C files of Objects
and Python subdirectories.

This implements things like `list[int]`,
which returns an object of type `types.GenericAlias`.
This object mostly acts as a proxy for `list`,
but has attributes `__origin__` and `__args__`
that allow recovering the parts (with values `list` and `(int,)`.

There is also an approximate notion of type variables;
e.g. `list[T]` has a `__parameters__` attribute equal to `(T,)`.
Type variables are objects of type `typing.TypeVar`.