doing a critical operation in C.  When fiddling the reference counting
we often need to

    1. unpack a 64bit string into a long
    2. add a delta value to the long
    3. pack the long into a 64bit string

_helper.incr() does these three steps in C.

This extension only works for Python 2.2, but Full.py has a Python
replacement for use with Python 2.1.