[GHC] #15210: Use ByteStrings for strings that don't use FastString's features

[GHC]

#15210: Use ByteStrings for strings that don't use FastString's features
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        Reporter:  sjakobi           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:  performance,
                                     |  strings
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #15157            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 Note that we also have `LitString` for literal strings. Otherwise I agree
 that a newtype-wrapped UTF-8 `ByteString` would make sense here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15210#comment:1>
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