Support using UCS-4 as the Py_UNICODE type:

Add configure option --enable-unicode.
Add config.h macros Py_USING_UNICODE, PY_UNICODE_TYPE, Py_UNICODE_SIZE,
                    SIZEOF_WCHAR_T.
Define Py_UCS2.
Encode and decode large UTF-8 characters into single Py_UNICODE values
for wide Unicode types; likewise for UTF-16.
Remove test whether sizeof Py_UNICODE is two.