Adding an official Tokio job executor

[jeffmoss]

I was reviewing #4130 for information on how to build a Tokio-based event loop that can run asynchronously and came to the conclusion that this is an area for improvement that would make the library great for light scripting tasks in my project, similar to Rhai (which doesn't do promises/futures like I want).

Is my understanding correct that utilizing tokio's timer wheel, or an even more sophisticated implementation like libuv employs, would make for an energy-efficient executor that can run virtually idle? My thinking was to start by adding a simple "tick" of 100ms or so granularity to ensure the timeout_jobs are eventually fulfilled, but this would be very crude.

Do you foresee advanced executors being external crates in the future, like a boa-tokio crate?

Just wanted to kick off a discussion and hear about design goals, future directions, etc.