A yield_now variant

`async_io::block_on` acts like `futures_lite::block_on` but runs async-io tasks during downtime. This is pretty useful until you need to write your own `block_on` implementation and it's no longer composable.

It'd be pretty useful to allow periodically yielding to async-io tasks from synchronous code. This is usually exposed in the form of a `yield_now` function.