Initial prototype for BiStream

[Brian Goetz]

> An awkward-looking BiIterator?
>
> public interface BiIterator<T, U> extends Iterator<BiValue<T, U>>

Yep, you've got the idea.  We may need a similar concept if we create 
specialized stream interfaces for primitives (which is the most logical 
place to hang the all-important sum() method.)  Bottom line: Iterator is 
really constraining, in addition to all its other challenges.