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

[roconnor at theorem.ca]

It appears that 95% of identifiers are between 2 and 21 (inclusive) 
characters long.  So partitionEithers is not unusually long according to 
this metric.

-- 
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.''