Proposal: add foldMapWithIndex to Data.Sequence

[David Feuer]

foldMapWithIndex :: Monoid m
     => (Int -> a -> m) -> Seq a -> m

This would match both Data.Map.foldMapWithKey and
Control.Lens.Indexed.ifoldMap, filling out the indexed fold menagerie.

Annoyingly, the straightforward implementation I've come up with is rather
slow when used with Endo to implement either foldlWithIndex or
foldrWithIndex. I suspect the polymorphic recursion inherent in these
sequences is blinding GHC's optimizer. Oh well.
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