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

[GHC]

#9588: Add `MonadPlus (Either e)` and `Alternative (Either e)` instances
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        Reporter:  hvr               |                   Owner:  hvr
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:  ⊥
       Component:  Core Libraries    |                 Version:
      Resolution:                    |                Keywords:  report-
                                     |  impact
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #10755            |  Differential Revisions:
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Comment (by ekmett):

 I've updated the description to note that the CLC approved the IO
 instances unconditionally, but expressed reservations about the Either
 instances.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9588#comment:9>
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