Implement iters.iterate

Influenced by `iterate` in Clojure. I tried to find something similar in Python, but I couldn't. Please let me know if I missed it.

Basically, `iters.iterate(f, x)` returns a lazy sequence of x, f(x), f(f(x)) etc. It's similar to `iters.accumulate` but it takes an initial value, `x`, instead of an iterable.