Special case the "s#" PyArg_Parse() token for Unicode objects:

Special case the "s#" PyArg_Parse() token for Unicode objects:
"s#" will now return a pointer to the default encoded string data
of the Unicode object instead of a pointer to the raw UTF-16
data.

The latter is still available via PyObject_AsReadBuffer().

The patch also adds an optimization for string objects which is
based on the fact that string objects return the raw character data
for getreadbuffer access and are always single-segment.