[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
           Keywords:  performance,   |  Operating System:  Unknown/Multiple
  strings                            |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #15157
Differential Rev(s):                 |         Wiki Page:
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 The first two candidates that I have in mind are SourceText and
 StringLiteral's sl_fs field.

 There might be a point in creating a simple wrapper for UTF8-encoded
 ByteString. HsDocString could use that one too.

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