Multiset interface improvements

I find the name `multiset` as it makes me think of multiple sets. What we really made was a hash table that was used to count the occurence of hashes, so this mostly renames `hash_multiset` to `hash_counting_table`. I also felt the names `push` and `pop` were a bit weird, seeing as we're basically just incrementing and decrementing a counter so they're now `inc` and `dec`. Finally, seeing as I got rid of `set` from the name, I renamed the main argument to the interface functions `tab`.