Add iterate and iterateIndexed methods for Collections in the stdlib

[stackoverflow]

No description provided.

[HT154]

Looks good, just a couple questions

[bioball]

Are there other libraries that also call this iterate?

I also wonder if we should use a clojure-style "return a reduced value" to short-circuit?

In addition to fold, we also have foldBack and reduce. It'd be strange if you can short-circuit here, but not in the other two methods.

[stackoverflow]

Yeah, that's a good point. I can change the type signature of the folding function to return Element|Reduced<Element> and just change the original functions, instead of creating new ones.

Edit: scratch that. I went with the Clojure approach of having a short-circuiting function that you can call from within any folding/reducing function.