No foldMap in Vector.Unboxed or Vector.Primitive #263

Both Data.Vector.Primitive and Data.Vector.Unboxed do not have a foldMap function. There is no particularly compelling technical reason this doesn't exist. It has the following implementation for example:

foldMap :: forall m a . (Unbox a, Monoid m) => (a -> m) -> Vector a -> m
{-# INLINE foldMap #-}
foldMap f = foldr (mappend . f) mempty

foldMap :: forall m a. (Prim a, Monoid m) => (a -> m) -> Vector a -> m
{-# INLINE foldMap #-}
foldMap f = foldr (mappend . f) mempty

Note that this is the default implementation in the Foldable typeclass. The main downside to adding this definition is that is adds a potential namespace clash but both of these modules are already drowning in such namespace clashes and so I don't see this as a serious objection.