Collectors update

[Brian Goetz]

I've done some refactoring in Collectors to address some issues, 
including approachability issues that were raised by Kevin.  See if you 
like them.

1.  Rename to more verby names.  We now have
   groupingBy
   groupingReduce
   joiningWith
   partitioningBy

instead of groupBy/mappedTo/partition.  This is designed to make it more 
clear what is going on when you read code like:

   stream.collect(groupingBy(Foo::getBar))

by making it clear that groupingBy is not an action but a way of 
describing the desired collecting.

2.  Break groupBy into two groups, groupingBy and groupingReduce.  While 
the distinction is somewhat arbitrary since groupBy *is* a reduce, it is 
now more consistent with the method naming on Stream.

3.  Rename mappedTo(T->U) to joiningWith.

4.  Rejigger Partition to return an array again, with an explicit lambda 
(which will likely be an array ctor ref) to make the array.  Eliminated 
the silly Partition class.

5.  Add more tests -- we now have full test coverage on groupingXxx but 
not yet the others.