[GHC] #974: Add unzipEithers, lefts, rights to Data.Either

[roconnor at theorem.ca]

Everyone seems satified with the name unzipEithers.  There have been 
suggestions for

  partitionEither :: (a -> Either b c) -> [a] -> ([b], [c])

but I suggest they be pursued in a different trac.

I suggest accepting the patch (not that I have any authority) on Nov. 17 
if there are no further comments.

-- 
Russell O'Connor                                      <http://r6.ca/>
``All talk about `theft,''' the general counsel of the American Graphophone
Company wrote, ``is the merest claptrap, for there exists no property in
ideas musical, literary or artistic, except as defined by statute.''