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.
Victor Stinner