BiCollector

[Brian Goetz]

> collectingToBoth

This one is actually both evocative of what the method does, and in the 
spirit of the existing naming conventions (collectingAndThen.)

An n-ary version could just be called `collectingTo`, where it is passed 
a varargs of Collector.  Could we get away with collectingTo for a 
binary version as well?  The existence of the "combiner" function might 
make that a stretch, but I prefer `collectingTo` to `collectingToBoth`.


(I still like `distributing` too.)