[GHC] #9588: Add `MonadPlus {IO, Either e}` and `Alternative (Either e)` instances

[GHC]

#9588: Add `MonadPlus {IO,Either e}` and `Alternative (Either e)` instances
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        Reporter:  hvr               |                   Owner:  hvr
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Core Libraries    |                 Version:
      Resolution:                    |                Keywords:  report-
Operating System:  Unknown/Multiple  |  impact
 Type of failure:  None/Unknown      |            Architecture:
      Blocked By:                    |  Unknown/Multiple
 Related Tickets:                    |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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Comment (by ekmett):

 We should at least split this up into the easy and the hard cases and bite
 off the `IO` instances for now, and we can explicitly punt on the `Either
 e` case until someone comes up with a better idea.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9588#comment:6>
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