compressor: use unsigned long long for source size
compressor: use unsigned long long for source size

Because on 32-bit platforms, Py_ssize_t overflows. The actual type
used by the zstandard API is unsigned long long. So if nothing else
we're consistent.

There may still be some coercion warnings on some platforms. Let's
see what happens...
indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard