Improve rule matching

When doing constraint simplification on the LHS of a rule,
we *don't* want to do superclass commoning up.  Consider

	fromIntegral :: (Integral a, Num b) => a -> b
	{-# RULES "foo"  fromIntegral = id :: Int -> Int #-}

Here, a=b=Int, and Num Int is a superclass of Integral Int. But we *dont*
want to get

	forall dIntegralInt.
	fromIntegral Int Int dIntegralInt (scsel dIntegralInt) = id Int

because the scsel (super class selection) will mess up matching.
Instead we want

	forall dIntegralInt, dNumInt.
	fromIntegral Int Int dIntegralInt dNumInt = id Int


TcSimplify.tcSimplifyToDicts is the relevant function, but I had
to generalise the main simplification loop a little (adding the
type WantSCs).