Add instances for Mutable t.f. for arrays from primitive (#473)

While arrays from primitive could not have `Vector`/`MVector` instances,
`Mutable` is well defined for them. It's useful on its own and could be used
with another pair of type classes.

vector/src/Data/Vector/Generic/Base.hs

@@ -28,6 +28,9 @@ module Data.Vector.Generic.Base (
 import           Data.Vector.Generic.Mutable.Base ( MVector )
 import qualified Data.Vector.Generic.Mutable.Base as M
 import           Data.Vector.Fusion.Util (Box(..), liftBox)
+import qualified Data.Primitive.Array as Prim
+import qualified Data.Primitive.SmallArray as Prim
+import qualified Data.Primitive.PrimArray as Prim
 
 import Control.Monad.ST
 import Data.Kind (Type)
@@ -36,6 +39,11 @@ import Data.Kind (Type)
 -- the state token @s@. It is injective on GHC 8 and newer.
 type family Mutable (v :: Type -> Type) = (mv :: Type -> Type -> Type) | mv -> v
 
+type instance Mutable Prim.Array      = Prim.MutableArray
+type instance Mutable Prim.SmallArray = Prim.SmallMutableArray
+type instance Mutable Prim.PrimArray  = Prim.MutablePrimArray
+
+
 -- | Class of immutable vectors. Every immutable vector is associated with its
 -- mutable version through the 'Mutable' type family. Methods of this class
 -- should not be used directly. Instead, "Data.Vector.Generic" and other