add traverse/1 to Stream

[ckampfe]

This PR adds Stream.traverse/1. See proposal here: https://groups.google.com/forum/#!topic/elixir-lang-core/B5e8mWsQto0

[josevalim]

Your implementation handles only lists and maps, and not all enumerables. A function in Stream would need to consider all enumerables but the issues is, doing so is quite hard, as you can only know if something is an enumerable or not if try to invoke the protocol in it, and that can be potentially quite expensive.

So it is likely this is better implemented in your application with the constraints that make sense for your application, rather than in Elixir.

In any case, thanks for the PR!

[ckampfe]

Makes sense, thank you!