groupBy / reduceBy

[Brian Goetz]

> the other problem with reduceBy is that the combiner is only needed for
> the parallel case but not for the serial one.

Which is true for other nonhomogeneous reducer forms as well.

Does this bother us?  We've already made the decision to have one Stream 
type, rather than separate ones for serial and parallel.  .  I think 
this is the right call, but if people disagree, let's hear it.