This commit fixes an issue with the rust async support where when a
yield-loop was detected it suspended with `CALLBACK_CODE_YIELD` which
meant that it wasn't possible to deliver async events for other async
operations in progress. The fix is to instead return
`CALLBACK_CODE_POLL` with the waitable-set that has all the waitables
inside of it. This enables delivery of async events from that set to
ensure that if the yielding task is waiting on something else that it
gets delivered.

This also refactors a few things here and there, such as
`CALLBACK_CODE_*`, to be less error prone. Support was added for
composing 3+ components with `wasm-compose` since `wac` doesn't yet
use the same `wasm-tools`.