AbstractSet is functor

[kleinschmidt]

Currently, Base.Set and possibly other AbstractSets are treated as normal
structs. Since Base.Set{K} wraps a Dict{K,Nothing}, the Set constructor
treats its input as an iterable of values, and Dict iterates Pairs, Sets
end up as Set{Pair{T,Nothing}} after walking.

This adds tests to ensure that Set and OrderedSet (on behalf of all
AbstractSets everywhere) survive fmap.

I'm not 100% sure this is the right way to handle these (generally) un-ordered,
not-natively-mapable collections (I think you might get surprising results
using fmap with multiple arguments for instance. I can work aroudn this with
some lite type piracy but wanted to open this up for discussion anyway!