More `concatMap` functions (indexed and mutable)

[toyboot4e]

Is it good idea to add the following to Data.Vector.Generic? Thank you.

1. iconcatMap

iconcatMap :: (Vector v a, Vector v b) => (Int -> a -> v b) -> v a -> v b
{-# INLINE iconcatMap #-}
iconcatMap f = unstream
               . Bundle.concatVectors
               . Bundle.inplace (S.map (uncurry f) . S.indexed) id
               . stream

2. concatMapM

concatMapM :: (Monad m, Vector v a, Vector v b) => (a -> m (v b)) -> v a -> m (v b)
{-# INLINE concatMapM #-}
concatMapM f = unstreamM
               . MBundle.concatVectors
               . MBundle.mapM f
               . MBundle.fromVector

3. iconcatMapM

iconcatMapM :: (Monad m, Vector v a, Vector v b) => (Int -> a -> m (v b)) -> v a -> m (v b)
{-# INLINE iconcatMapM #-}
iconcatMapM f = unstreamM
                . MBundle.concatVectors
                . MBundle.mapM (uncurry f)
                . MBundle.indexed
                . MBundle.fromVector

[toyboot4e]

If these functions don't look so useful, feel free to close!

I occasionally want to use them in such situations:

• iconcatMap: Creating graph edge with edge indices
• concatMapM: Push-based dynamic programming
• iconcatMapM: Same (push-based dynamic programming)