This patch fixes cPickle.c for 64-bit platforms.

- The false assumption sizeof(long) == size(void*) exists where
PyInt_FromLong is used to represent a pointer. The safe Python call
for this is PyLong_FromVoidPtr. (On platforms where the above
assumption *is* true a PyInt is returned as before so there is no
effective change.)

- use size_t instead of int for some variables