Make HsDocString a newtype of ByteString

Authored by sjakobi on May 30 2018, 7:38 PM.

Description

Make HsDocString a newtype of ByteString

Docstrings don't profit from FastString's interning, so we switch to
a different type that doesn't incur this overhead.

Updates the haddock submodule.

Reviewers: alexbiehl, bgamari

Reviewed By: alexbiehl, bgamari

Subscribers: rwbarton, thomie, mpickering, carter

GHC Trac Issues: Trac #15157

Differential Revision: https://phabricator.haskell.org/D4743