Support latest WIT changes for P3

This issue is meant to track the changes needed to bring the `wac` language up-to-date with the latest in the `wit` language to support wasip3. As far as i'm aware the major change here is the addition of the `async` keyword in wit and some name mangling with look ups (e.g. `[async]foo`). 

> NOTE: `wac` has its own implementation of the wit language. 