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

[roconnor at theorem.ca]

I've removed the controvertial isLeft, fromLeft, isRight, fromRight from 
the trac, and added lefts and rights. The controvertial additions can be 
put in a new trac. Let us focus on the the noncontrovertial additions 
first.

I've also renamed splitEithers into unzipEithers, which I think is better.

A new patch is available on the trac: 
<http://hackage.haskell.org/trac/ghc/ticket/974>

-- 
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.''