Let me make sure I understand:
Cython automatically types th index as ``Py_ssize_t``.  IIUC, the issue is that ``Py_ssize_t`` is 32-bit or 64-bit depending on the platform, but we'd like our indices to be 64-bit everywhere just to be safe.  ``npy_intp`` is always 64-bit, so it would be a better choice for loop indices.  Is that correct?