[GHC] #12970: Add default implementation for Bits.bitSize

[GHC]

#12970: Add default implementation for Bits.bitSize
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        Reporter:  txnull            |                Owner:
            Type:  feature request   |               Status:  upstream
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:
      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 hvr):

 it may make sense to also consider a DefaultSignature-based default impl
 for `bitSizeMaybe` inheriting from `FiniteBits(finiteBitSize)`

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12970#comment:2>
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