Optimize mapping keys and values?

For `Data.Map`, at least, it should be pretty easy to fuse `mapKeysMonotonic` with `map`, and probably even with `mapMaybeWithKey`. For `mapKeys` more generally, we might want to take another look at combos with list operations. For example, `toList . mapMaybeWithKey f` could be rewritten to convert to a list first and then `mapMaybe`.