[GHC] #8894: Clean up `Coercible` plumbing

[GHC]

#8894: Clean up `Coercible` plumbing
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       Reporter:  nomeata           |             Owner:
           Type:  task              |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.9
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Currently, the way `Coercible` is exported (with an data type in
 `GHC.Types` and a built-in !TyCon exported in `GHC.Prim`) works, but is
 confusing and should be cleaned up. Probably by exporting it directly from
 `GHC.Types`.

 Care needs to be taken that ghci and haddock display the data from
 modified built-in !TyCon; when I tried that a while ago it was displayed
 as a regular data type by one of them. If that is still the case and I
 didn't just do a mistake back then, this probably needs to be understood
 and fixed.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8894>
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