Is there a nice (built-in) way to apply Replace-style rules to key-value pairs in an association? Something like

```
SomeFunction[<| "id" -> 3, "freq" -> 4 |>, "freq" -> #freq+1 &]
(* should produce <| "id" -> 3, "freq" -> 5 |> *)

```

This could also be used to add computed keys, like

```
SomeFunction[..., "density" -> #mass / #volume &]

```

This would be really useful when combining with `Query` and `Dataset`.