c4f9ef2e2e
This patch renames `dispatch_and_get_new_users` to `dispatch`. This response processor no longer returns the new user IDs, but they are written in a mutable reference argument. This argument is typed by a new private trait: `NewUsers`. This trait is implemented on `BTreeSet<OwnedUserId>` and on `()`. It helps to avoid allocations when the new users are not needed.