Observable from Func0<Enumeration>

dealing with `HttpServletRequest` API that returns `Enumeration`s and want to get out of there as fast a possible.  The `Func0` emulates the `Iterable` interface so subsequent subscribes get a fresh `Enumeration`.
I still don't like that we are adding such a base type as `Enumerable`. This feels wrong, like the type of thing we'll regret but be stuck with.

I suggest this as a contrib module for now so that it's not baked into core.