[GHC] #15345: Duplication between `CoreSubst` and `SimplEnv` is very unfortunate

[GHC]

#15345: Duplication between `CoreSubst` and `SimplEnv` is very unfortunate
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        Reporter:  mpickering        |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 And I found another function which is rather like `substIdBnd`,
 `CoreOpt.subst_opt_id_bndr`. I don't think there should be four functions
 doing this very similar task of applying a substitution to a binder.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15345#comment:1>
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