Implement Wasmtime's new API as designed by RFC 11. This is quite a large commit which has had lots of discussion externally, so for more information it's best to read the RFC thread and the PR thread.
* ctx parameter no longer accepted by wiggle::from_witx macro.
* optional async_ parameter specifies which functions are async.
* re-export async_trait::async_trait, so users don't have to take a dep.