Can we make the Map.Merge API more expressive?

I wanted to demonstrate `partitionKeys` recently (https://github.com/haskell/containers/pull/975#issuecomment-2417976839) and realized that the public Map.Merge API is not expressive enough for it.

What I need:

```hs
wm1 :: WhenMissing Pair k a a
wm1 = WhenMissing (\t -> Pair empty t) (\_ x -> Pair Nothing (Just x))
```

Best I can do with the public API:

```hs
wm1 :: WhenMissing Pair k a a
wm1 = traverseMaybeMissing (\_ x -> Pair Nothing (Just x))
```

which is terribly inefficient! (O(1) vs O(n))

Is there a safe way to allow such use cases?

Start by reading the public Map.Merge API, especially WhenMissing and traverseMaybeMissing, and review the partitionKeys example linked in the issue. Determine whether a safe public API can express the shown wm1 behavior without the O(n) traversal; done means the use case is supported with the stated efficiency and its safety is clear.

Written by the indexing model from the issue text.