Add LazyPowerSet and LazyKSubsets

Add LazyPowerSet and LazyKSubsets

Two new classes were added that act effectively
as convenience methods around creating subset
iterators. The classes themselves act mostly
like iterators but are instead light-weight
"containers" which can be iterated over. Effectively,
they are contains which "contain" the entire
collection of subsets (of size k respectively) of
some collection without actually storing that collection
in memory.