Ergonomics of the Guest trait implementation from the generated Rust bindings from WIT

[greenhat]

The Guest trait in the generated Rust bindings defines the exported interface of the WIT world. It's the Rust variant of the exported WIT interface, and the user needs to implement it.
The trait name Guest is rooted in the host/guest tandem of the Wasm CM. It's hard-coded in the wit-bingen.
We could make it configurable and try to push it to the upstream.

Another option would be to try to hide the impl Guest behind the macro on the user's account struct. We need to somehow preserve the DX of adding new functions to the WIT interface, e.g. "implement missing..." action, etc.

Investigate cxx crate and bridge macro along with generating the WIT interface from user's Rust code and hiding the WIT interfaces and their generated Rust bindings completely.