executor independent spawning

It looks like originally the `Context` was intended to have an executor attached to it so that you can spawn tasks in an executor independent way. With `async-io` we no longer need to rely on a specific runtime and wrapping the std types, which is a hughe gain for writing async code that works everywhere. However it is very useful sometimes to be able to spawn a background task. There doesn't seem to be a way to do this without either passing around a "spawner" or deciding on an executor. Is there likely going to be a `std::task::spawn` that executors can hook in to?