stream() / parallelStream() methods

[Brian Goetz]

> Here's the other issue this raises.
>
> To my knowledge there's no Streamable<T> interface defined.

Right.  Earlier drafts had one (ask Doug to recount the "OMG so many 
interfaces" horror of Iteration 2), and since then we've been working 
really hard to eliminate each incremental public type, as each adds API 
surface area.  I think we've been really successful at this; I'd hate to 
slide backwards.

> Maybe it
> wasn't needed; I'm not sure.  But once Iterable looks like this, now
> Iterable becomes the new Streamable.  If you support a stream(), you'll
> implement Iterable to expose that fact.  This is a little bit weird.
>   I'm undecided on how big a problem it would be, but overall,
> Streamable<T> seems like a pretty normal thing to have.

Leading question: if everything that is Iterable is effectively 
Streamable (because Iterable has a stream()) method, and everything 
Streamable is effectively Iterable (because you can turn a Spliterator 
into an Iterator), aren't they then the same abstraction?