[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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Description changed by ekmett:

Old description:

> The following 2 instances are currently Orphans in `transformers` but
> shall be defined in `base` instead:
>
> {{{#!hs
> instance MonadPlus (Either e)
> instance Alterantive (Either e)
> }}}
>
> This proposal by SPJ already passed the CLC back in April and only needs
> implementing. I'll submit a patch shortly to Phab

New description:

 The following 2 instances are currently Orphans in `transformers` but
 could be defined in `base` without instead:

 {{{#!hs
 instance Error e => MonadPlus (Either e)
 instance Error e => Alterantive (Either e)
 }}}

 This would, however, require us to move `Error` from `transformers` into
 base, which may be a controversial move.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9588#comment:8>
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