This PR rewrites the default serial port implementation to avoid relying on the DataReceived event. Besides being prone to deadlock on dispose as observed in #1437, the event callbacks for serial port are not available in several implementations, including Mono and Unity3D.

The new implementation uses directly the serial port stream through dedicated service threads that perform blocking reads. These can be interrupted asynchronously by directly calling Dispose from a separate thread, so they afford synchronized disposal without any additional logic. Furthermore, this approach establishes an initial exploration for later generalizing asynchronous line reads over any stream, which can be useful for streaming data from files, network sockets and other data sources.