Better documentation for sets
Due to the proliferation of type classes related to sets, finding things is extremely difficult. I almost convinced myself std++ doesn't have
list_to_set (it's in base and doesn't depend on
SemiSet, so my searches all failed) or
set_map (it's not an instance of
FMap due to #21).
It would be good to provide a short introduction to the overall structure of sets in std++, including the typeclasses, notations, and important instances.