Add iterate, from Haskell prelude.

http://zvon.org/comp/r/ref-Haskell.html#Functions~Prelude.iterate

It seems a bit odd to not have iterate in the same place as the fold* functions, but given that accumulate and reduce aren't in the same place, I think this goes here, while they go in more-functools (or an updated version of the old functional) or something.