Generalised transformation functions.

Consider adding the following functions:

## `withMap`

```hs
withMap
:: MonoidNull v2
=> (Map k1 v1 -> Map k2 v2)
-> (MonoidMap k1 v1 -> MonoidMap k2 v2)
withMap f = fromMap . f . toMap
```

## `withMapKeys`

Applies a transformation that is guaranteed to neither create nor modify values, avoiding the need to perform a post-processing step that removes `null` values.

```hs
withMapKeys
:: (forall x. Map k1 x -> Map k2 x)
-> (MonoidMap k1 v -> MonoidMap k2 v)
withMapKeys f (MonoidMap m) = MonoidMap (f m)
```

See: 3deb419d221b90ad8593954758b083ad9ca7ff92