add stateless effectful unfold

[tpolecat]

This adds a method to unfold an effectful computation F[Option[A]] into Stream[F,A].

resolves #690

[fiadliel]

I'd usually write that kind of behaviour in this way (unNoneTerminate is the standard pipe combinator for terminating Option[_] streams):

Stream.repeatEval(f).through(pipe.unNoneTerminate)

But then I wouldn't write it that way in StreamDerived, which probably shouldn't depend on functions from pipe.

I'm not strongly against this extra method, but just asking if the addition pulls its weight, given available combinators...?

[tpolecat]

Ok, I didn't know about that pipe combinator. That works for me.