WIP - convert native integer values to smallest allowed Python integer object representation

When boxing a native long long or unsigned long long on Python 2, we should use the PyInt_* functions when the value is in range.  This PR solves issue #474.

However, it introduces two new helper functions to _helperlib.c, which causes pycc tests to fail with missing symbol issues.  Any idea @sklam how I should link _helperlib.c into pycc generated shared libraries?