Comment out most of Data.Bits and Data.Mod2
Since they rely on a bug in the unifier!
Check that explicitly given implicit args unify with inferred versions
Also exposed a unification bug which unified things which shouldn't be:
f x = f y does not imply x = y unless f is injective, and this was not
properly checked.

Fixes issue #127, as it caused a related problem in the coverage checker.