Fix a bug with an unassigned local variable

I was passing an unassigned size_t to PyArg_ParseTuple with the "s#"
format string. Unless you have PY_SSIZE_T_CLEAN defined, that format
string expects that you have passed an int. So I was passing the address
of an unassigned 64 bit value and the Python API was only writing to the
lower 32 bits. Problems occurred when that address had anything but
zeros stored in the higher 32 bits.

The code now defines PY_SSIZE_T_CLEAN, uses Py_ssize_t where it should
and also doesn't pass unassigned local variables to PyArg_ParseTuple et.
al.
Add static declarations to internal functions
Reference diesel_buffer directly in places

I was dereferencing Buffer->internal_buffer all over the place. It seems
more clear to just work with that directly.
cbuf.Buffer.feed optimization

If the term is ANY, don't actually buffer what is being fed in - return
it directly.

This seems like a safe optimization that even the pure-Python version
could take advantage of.