Performance week 25/6-29/6

[Simon Marlow]

Ok, we made some progress on performance last week, but there's still
plenty to do.  Here's a summary:

  - Simon PJ found an O(n^2) case in the simplifier that appears to
    be the cause of most, if not all, of the cases of GHC going out
    to lunch when compiling certain modules.  However, while testing
    the fix we discovered that it caused worse code to be generated
    for some programs, so we're still looking into this.

  - I added a hack in GHC's back end to do some lightweight buffering of
    the