Changes to ghc

Changes to ghc:
commit a4a79b5a04658ac542b1e07a6975b488fd589441
Author: Edward Z. Yang <ezyang@cs.stanford.edu>
Date:   Tue Jun 24 17:29:49 2014 +0100

    Describe signature mini-backpack.

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