[GHC] #9165: Export TimerManager from GHC.Event

[GHC]

#9165: Export TimerManager from GHC.Event
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       Reporter:  basvandijk               |             Owner:
           Type:  task                     |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  libraries/base           |           Version:  7.8.2
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Easy (less than 1 hour)  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 [http://hackage.haskell.org/package/base-4.7.0.0/docs/GHC-Event.html
 GHC.Event] exports `getSystemTimerManager` but not the actual
 `TimerManager` type. I propose to export it so that the haddocks are
 properly linked and users can actually refer to `TimerManager` in their
 type signatures.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9165>
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