[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 simonmar):

 @simonpj the .gitmodules changes are because the diff on phabricator
 includes changes to submodules, and to avoid having to push to the
 upstream for those submodules it's necessary to fork the repos and point
 the .gitmodules file at your fork for the purposes of uploading the patch
 to phabricator.  The idea is that before pushing the final patch, you
 revert the .gitmodules changes and push the necessary submodule changes to
 the relevant upstream repos.

 I've been encouraging @bgamari and @austin to document this (slightly
 complex) workflow on the wiki, it really needs to go here:
 https://ghc.haskell.org/trac/ghc/wiki/WorkingConventions/Git/Submodules

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5218#comment:55>
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