Add monadic versions of the unfoldr functions

[hansroland]

Currently vector-sized implements only pure, non-monadic version of the unfoldrfunctions, eg in the unboxed module:

• unfoldrN :: forall n a b. (KnownNat n, Unbox a) => (b -> (a, b)) -> b -> Vector n a
• unfoldrN' :: forall n a b p. (KnownNat n, Unbox a) => p n -> (b -> (a, b)) -> b -> Vector n a

The normal vector package has also monadic versions like unfoldrM, unfoldrNM and unfoldrExactNM.

It would be nice to have them also in this library, eg for the unboxed module:

• unfoldrNM :: forall m n a b. (KnownNat n, Unbox a, Monad m) => (b -> m (a, b)) -> b -> m (Vector n a)
• unfoldrNM' :: forall m n a b p. (KnownNat n, Unbox a, Monad m) => p n -> (b -> m (a, b)) -> b -> m (Vector n a)