* Update the source path of the pegen target within the Windows regen project.
Change the path to Windows path formats.

* Use the more reliable SetEnv task for Cpp Projects in MSBuild.

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.

Update the "Makefile.pre.in" template and the "PCbuild/lib.pyproj" with the files in "Lib/test/test/test_peg_generator" so they get correctly installed along the rest of the standard library.

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>

Icon author: Andrew Clover, bpo-1490384

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.