make Data.Vector.Generic.Base.Mutable injective (#160)

* make Data.Vector.Generic.Base.Mutable injective
* Add test, use __GLASGOW_HASKELL_

Author: strake

Data/Vector/Generic/Base.hs

@@ -1,5 +1,9 @@
 {-# LANGUAGE Rank2Types, MultiParamTypeClasses, FlexibleContexts,
              TypeFamilies, ScopedTypeVariables, BangPatterns #-}
+{-# LANGUAGE CPP #-}
+#if __GLASGOW_HASKELL__ >= 800
+{-# LANGUAGE TypeFamilyDependencies #-}
+#endif
 {-# OPTIONS_HADDOCK hide #-}
 
 -- |
@@ -24,9 +28,13 @@ import qualified Data.Vector.Generic.Mutable.Base as M
 import Control.Monad.Primitive
 
 -- | @Mutable v s a@ is the mutable version of the pure vector type @v a@ with
--- the state token @s@
+-- the state token @s@. It is injective on GHC 8 and newer.
 --
+#if MIN_VERSION_base(4,9,0)
+type family Mutable (v :: * -> *) = (mv :: * -> * -> *) | mv -> v
+#else
 type family Mutable (v :: * -> *) :: * -> * -> *
+#endif
 
 -- | Class of immutable vectors. Every immutable vector is associated with its
 -- mutable version through the 'Mutable' type family. Methods of this class