Add routes and serve alternatives that return Future
Current `routes` and `serve` don't expose the future they received from `asynchttpserver.serve`. This means that API user can't handle corresponding errors locally - they are propagated all the way to `poll`/`waitFor`/`runForever`.
Since making `routes` and `serve` return a `Future` would break a lot of code (all of it, I imagine), it would make sense to add new functions that return futures rather than modifying the existing ones.