Hamster::Enumerable#group_by uses Hamster::Hash, but in some cases people may not
have actually loaded Hamster::Hash.

Thanks to Dov Murik for noticing this bug.
Hamster::Enumerable methods work on Hash, even when other Hamster collections have not been loaded
Removed unneeded #null? alias from Deque
Removed unneeded #null?, #diff, #intersect, #nub, #uniq, #remove_duplicates aliases from SortedSet