[3.5] bpo-30708: Add private C API function _PyUnicode_AsWideCharString(). (GH-2285...

[3.5] bpo-30708: Add private C API function _PyUnicode_AsWideCharString(). (GH-2285) (GH-2443)  (#2448)

And use it instead of PyUnicode_AsWideCharString() if appropriate.

_PyUnicode_AsWideCharString(unicode) is like PyUnicode_AsWideCharString(unicode, NULL), but
raises a ValueError if the wchar_t* string contains null characters.
(cherry picked from commit e613e6add5f07ff6aad5802924596b631b707d2a).
(cherry picked from commit 0edffa3073b551ffeca34952529e7b292f1bd350)