Remove unnecessary language extensions

Attempting to abide by the Principle of Least Power. Also: the smaller the
headers in each module are, the happier I am.

src/Types.hs

@@ -1,5 +1,3 @@
-{-# LANGUAGE EmptyDataDecls #-}
-{-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MultiParamTypeClasses #-}