No overflow check for long long

[robertwb]

Cython uses PyInt_AsUnsignedLongLongMask to convert to a long long, but this does not check to see if the result fits.

http://docs.python.org/api/intObjects.html

Perhaps we should create our own __pyx_PyInt_AsLongLong which performs this check.

[robertwb (robertwb)]

Overflow checks seem to be incorrect for types smaller than a long too.