[GHC] #15157: Use a better string type (ByteString?!) for HsDocString

[GHC]

#15157: Use a better string type (ByteString?!) for HsDocString
-------------------------------------+-------------------------------------
           Reporter:  sjakobi        |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 `HsDocString` is currently defined as

 {{{
 newtype HsDocString = HsDocString FastString
 }}}

 Alex Biehl and hvr mentioned that due to `FastString`'s (interning?)
 overhead it's not a good type to store haddocks. They suggested using
 `ByteString` instead.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15157>
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