[GHC] #5556: Support pin-changing on ByteArray#s

[GHC]

#5556: Support pin-changing on ByteArray#s
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        Reporter:  pumpkin           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.2.1
      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 Mathnerd314):

 #12059 adds an `isPinnedByteArray#` primop, but there is still no zero-
 copy method of pinning / unpinning.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5556#comment:15>
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