Add instances for Functor, etc., for Set

[lukaszcz]

• To avoid conversions to/from List
• Also reimplement the inefficient implementation of Foldable, etc., which uses conversions to/from list under the hood

[paulcadman]

Haskell Set doesn't have a Functor instance.

The reason given is that the Functor law:

fmap (g . f) = fmap g . fmap f

can be violated. On the right duplicates are removed using f and then g . Whereas on the left duplicates are removed using g only.

I'm not sure if this is an issue for our Set implementation.

[lukaszcz]

Right, forgot about that. But we should have Set.map at least.

[heueristik]

I also have an implementation of isSubset. This should be in stdlib and probably should be improved.

    isSubset {A} {{Ord A}} (sub sup : Set A) : Bool := all (x in Set.toList sub) Set.isMember x sup;