Diffcheck

@gridaphobe this *should* fix that particular bug with diffcheck, that you mention in:

https://github.com/ucsd-progsys/liquidhaskell/blob/master/docs/slides/HS2014/ByteString-BAD-DIFFCHECK.hs#L55-L59

but I have left it as a separate branch / PR in case you want to go over your demo one more time using this "FIXED" branch just to make sure I haven't *broken* something new...