Roman produced programs involving associated types that did not optimise well.
His programs were something like this:

  data family T a
  data instance T Int = MkT Bool Char

  bar :: T Int -> Int
  bar t = t `seq` loop 0
	where
	  loop = ...

You'd think that the `seq` should unbox 't' outside the loop, since
a (T Int) is just a MkT pair.  

The most robust way to make this happen is for the simplifier to understand
a bit about type-family instances.   See 
	Note [Improving seq]
in Simplify.lhs.  We use FamInstEnv.topNormaliseType to do the interesting
work.

To make this happen I did a bit of refactoring to the simplifier
monad.

I'd previously done a very similar transformation in LiberateCase, but it
was happening too late.  So this patch takes it out of LiberateCase as
well as adding it to Simplify.