Changes to ghc

Changes to ghc:
commit 7a783746972561565eacea634512c3888ded6ec2
Author: Edward Z. Yang <ezyang@cs.stanford.edu>
Date:   Mon Jun 23 17:26:10 2014 +0100

    More updates to Backpack impl docs.

    Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>