Make xrange more Py_ssize_t aware, by assuming a Py_ssize_t is always at

Make xrange more Py_ssize_t aware, by assuming a Py_ssize_t is always at
least as big as a long. I believe this to be a safe assumption that is being
made in many parts of CPython, but a check could be added.

len(xrange(sys.maxint)) works now, so fix the testsuite's odd exception for
64-bit platforms too. It also fixes 'zip(xrange(sys.maxint), it)' as a
portable-ish (if expensive) alternative to enumerate(it); since zip() now
calls len(), this was breaking on (real) 64-bit platforms. No additional
test was added for that behaviour.