Declarations of Windows-specific auxilary functions need Windows types
from windows.h. Instead of including windows.h in Python.h and making
it available to all Windows users, it is simpler and safer just move
declarations to the single file that needs them.
_Py_char2wchar() callers usually need the result size in characters. Since it's
trivial to compute it in _Py_char2wchar() (O(1) whereas wcslen() is O(n)), add
an option to get it.
* Don't define _Py_wstat() on Windows, Windows has its own _wstat() function
with a different API (the stat buffer has another type)
* Include windows.h
* _Py_fopen() and _Py_stat() come from Python/import.c
* (_Py)_wrealpath() comes from Python/sysmodule.c
* _Py_char2wchar(), _Py_wchar2char() and _Py_wfopen() come from Modules/main.c
* (_Py)_wstat(), (_Py)_wgetcwd(), _Py_wreadlink() come from Modules/getpath.c