Changes to ghc

Changes to ghc:
commit 5c904ba055147e0a71d5b200c8886ef0b1a47794
Author: Joachim Breitner <mail@joachim-breitner.de>
Date:   Sun Nov 24 19:38:08 2013 +0000

    Mention that the user has to import GHC.Exts for Constraint

    (required judging from confusion on #haskell right now.)