Rewrite scheduler to be an async runtime

[mkaput]

I see a great opportunity to make your own scheduler based on smol, i.e. on primitives from this executor that are exposed as separate packages. That would be great; one could make tasks that are interrupted, or those that jump into a synchronous thread when needed.

Useful crates:

• https://github.com/smol-rs/smol/blob/321ac3b9c45d4e286949132cd8b138cb50fe44fb/src/spawn.rs#L10-L64
• https://docs.rs/async-executor/latest/async_executor/
• https://docs.rs/async-task/latest/async_task/
• https://docs.rs/event-listener/latest/event_listener/
• https://docs.rs/futures-lite/latest/futures_lite/index.html