[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 #12160     |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 Sadly.

 The current Error based instance in transformers is somewhat lacking in
 this and other regards.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9588#comment:12>
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