[GHC] #5218: Add unpackCStringLen# to create Strings from string literals

[GHC]

#5218: Add unpackCStringLen# to create Strings from string literals
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        Reporter:  tibbe             |                Owner:  thoughtpolice
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.0.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5877 #10064      |  Differential Rev(s):  Phab:D2443
       Wiki Page:                    |
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Comment (by bgamari):

 Regarding comment:56,

 I don't think we would want to give every string literal the overhead of a
 full heap object. Even just adding a length as is proposed in this ticket
 is adding a significant binary size overhead for small strings, which
 occur quite often. The additional few words that a `ByteArray` closure
 would bring is far too expensive to stomach.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5218#comment:57>
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