Add `spanAntitoneM`

`spanAntitone :: (k -> Bool) -> Map k a -> (Map k a, Map k a)` is a handy way to "bisect" arbitrary data in the `git bisect` sense. Just stick your data in a `Map`, then use `spanAntitone` to find the crossover point.

Sometimes the antitone predicate cannot be defined purely though (maybe it involves interactivity, or consulting a database, or whatever), and in such cases I feel like it'd be handy to have this slight variation:

```haskell
spanAntitoneM :: Monad m => (k -> m Bool) -> Map k a -> m (Map k a, Map k a)
```

The rule for the predicate can still be specified as `j < k implies (>=) <$> p j <*> p k == f True`, for some `f :: forall x. x -> m x` (roughly, `(>=) <$> p j <*> p k` always "resolves" to `True`).