More generic `FMap` type class that also supports collections

Currently, collection_map, which turns a set (e.g. a gset) with elements of type A into a set with elements of type B cannot be an instance of the FMap type class. Let's take a look at the FMap class to see why:

Class FMap (M : Type → Type) := fmap : ∀ {A B}, (A → B) → M A → M B.

The problem here is that M has type Type → Type while gset has type ∀ (A : Type) ``{Countable A}, Type. For most other set implementations we would have the same problem, e.g. if we were to have some kind of binary search trees, the type would be something like ∀ (A : Type) ``{TotalOrder A}, Type.

In fact, our generic mapping operation for FinCollection has the following type:

Definition collection_map `{Elements A C, Singleton B D, Empty D, Union D}
    (f : A → B) (X : C) : D :=
  of_list (f <$> elements X).

I know this is also a problem in Haskell where the fmap operation on sets (which are implemented as trees, and thus needs to have access to the order) is not an instance of the Functor type class, so it may be worth taking a look at what they're doing.