mergers

[Brian Goetz]

Collectors defines three merge functions:

   throwingMerger -- always throws
   firstWinsMerger -- takes first
   lastWinsMerger -- takes last

These are plain old BinaryOperators that can be used for Map.merge as 
well as the toMap collectors.

Someone commented that these look a little out of place in Collectors, 
and they are certainly not Collector-specific.  Is there a better place 
for them?