When matching against a GADT, some of the constructors may be impossible.
For example
	data T a where
          T1 :: T Int
          T2 :: T Bool
          T3 :: T a

        f :: T Int -> Int
        f T1 = 3
        f T3 = 4

Here, does not have any missing cases, despite omittting T2, because
T2 :: T Bool.

This patch teaches the overlap checker about GADTs, which happily
turned out to be rather easy even though the overlap checker needs
a serious rewrite.