update Rust generator and runtime to match async spec (#1233)

This updates async support for the Rust generator and runtime to match recent
(and not-so-recent) spec changes:

- Use explicit waitable-sets to wait for waitables
- Return packed (`CallbackCode`, waitable-set) tuples from exports and callbacks
- Remove `Event.DONE`, which no longer exists

I've tested this end-to-end with `wasip3-prototyping`.

Signed-off-by: Joel Dice <[email protected]>

Author: dicej