More on functional dependencies

My last commit allowed this:

	instance C a b => C [a] [b] where ...

if we have

	class C a b | a -> b

This commit completes the change, by making the 
improvement stages improve only the 'shape' of the second
argument of C.  

I also had to change the iteration in TcSimplify -- see
the comments in TcSimplify.inferLoop.