PyLong_FromSsize_t was incorrect when sizeof(size_t) > sizeof(long);

PyLong_FromSsize_t was incorrect when sizeof(size_t) > sizeof(long);
rewrite it so that it doesn't care about relative sizes of size_t,
long and long long.

The rewrite is modeled on PyLong_FromLong, instead of using
PyLong_FromByteArray;  this makes the algorithm simpler and
more direct, and possibly also slightly faster.