Return `long' objects when `int' is requested but the value won't fit.

Mostly, Python handles the error from the `int' conversion and falls
back to long, but there's something weird in iteration, where if you say

	for i in ...:
	  print '%d' % x

then the loop finishes and /then/ you get an exception for the overflow
from the failed conversion of x to an `int'.

Follow Python's actual behaviour: have `mp_tolong_checked' take an extra
argument indicating whether to throw an exception, and modify most of
the call sites to fall back to a conversion based on `mp_topylong'.