Prevent SOE when traversing streams of Futures.

This moves the evaluation of the first future passed to ap within the
future itself. Since Futures are eager, this will still be parallel, but
prevents SOEs when using traverse on Stream or List with Future.
Add test for SOE on Stream.traverse + Future.
Merge pull request #785 from tixxit/topic/no-soe-on-future-ap

Prevent SOE on Stream traverse with Future