[GHC] #15879: Make UniqDSet a newtype

[GHC]

#15879: Make UniqDSet a newtype
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           Reporter:  sgraf          |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #13114
Differential Rev(s):                 |         Wiki Page:
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 For the same reasons as in #13114 and Phab:D3146 we should make `UniqDSet`
 a newtype of `UniqDFM`.

 I wonder if we can also have a prettier `Outputable` instance this way.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15879>
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