Implement #each method and include Enumerable.

I have a production system which needs an efficient network coding of a Ruby set, and converts the set to an array at both ends. DumbNumbSet is great for the former, but only exposes `include?` for the latter.

So I've added a very simple `#each` method and added the Enumerable mixin, which solves this problem for me. Tests for `#to_a` and `#to_set` are included.

Thanks for your library!