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

[roconnor at theorem.ca]

I think that the type of lefts and rights should be generalized to

lefts :: (MonadPlus m) => m (Either a b) -> m a
rights :: (MonadPlus m) => m (Either a b) -> m b

Any concerns about this?  Are the names still acceptable?

-- 
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.''