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

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.