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

[GHC]

#8894: Clean up `Coercible` plumbing
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        Reporter:  nomeata           |            Owner:  nomeata
            Type:  task              |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.9
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #8904
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Comment (by nomeata):

 No, I was a bad boy and lazy and thought I thought of everyting. Fixing
 the test cases to use the proper import point `Data.Coerce`.

 I would have noticed myself quicker if CI was useful at the moment, but
 there are other failures there waiting to be fixed for a few days now :-(

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8894#comment:12>
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