Synchronous Io also uses the Io instance now. The coloring is no longer "is it async?" it's "does it perform Io"?

This allows library authors to write their code in a manner that's agnostic to the Io runtime the user chooses, synchronous, threaded, evented with stackful coroutines, evented with stackless coroutines.

Rust also allows writing async code that is agnostic to the async runtime used. Subsuming async under Io doesn't change much imo.