Implement Foldable[Stream]#foldLeft in terms of scala.Stream#foldLeft

This fixes SOE on Foldable[Stream].foldLeft on large streams.

As requested by @larsrh, I [posted on the mailing list](https://groups.google.com/forum/?fromgroups=#!topic/scalaz/6H34GUL6eSA) about this a couple of days ago, but there have been no responses yet.