Allow type families to use GADT syntax (and be GADTs)

We've always intended to allow you to use GADT syntax for
data families:
	data instance T [a] where
	  T1 :: a -> T [a]
and indeed to allow data instances to *be* GADTs
	data intsance T [a] where
	  T1 :: Int -> T [Int]
	  T2 :: a -> b -> T [(a,b)]

This patch fixes the renamer and type checker to allow this.