Hash counting table increment now returns count

The interfaces to `hash_counting_table_inc()` and `hash_counting_table_dec()` were asymmetrical, which didn't seem right. Even though we don't use the returned value for anything, it seemed correct to return the pre-incrementation value from `hash_counting_table_inc()` just as we return the pre-decrementation value from `hash_counting_table_dec()`.

This also includes some more variable renaming, a smidge more testing, and some docstrings.