Set + SortedSet with tests

[laher]

See #32 . As I mentioned there, I already wrote this earlier. It's really just a convenience wrapper, but it seems desirable IMO.

• Note that I probably wouldn't bother with the SetBuilder/SortedSetBuilder if I were implementing varargs as in List: varargs for Append,Prepend,NewList #33 . I haven't added them here yet, but I think it makes sense to.
• I also haven't added Union/Difference/Intersection/Complement, but it seems like something we'd want on a set.

For both those things, I figure it's worth discussing first. e.g. Do you even want a set to exist in the package?

[benbjohnson]

Yeah, I think Set and SortedSet are nice API sugar. The struct{} values are kinda lame from an API standpoint and these types could probably be optimized in the future to remove the values internally entirely.

[benbjohnson]

I went ahead and tagged your last 2 PRs into a release: https://github.com/benbjohnson/immutable/releases/tag/v0.4.2