Switch to WIT

[abrown]

The wasi-nn specification has been ported over from the old WITX syntax to the new WIT syntax (see WebAssembly/wasi-nn#17). This repository should switch over to using WIT-based tooling. I see a couple of ways this could work:
a. we replace the generated bindings in this repository with bindings generated by wit-bindgen
b. we remove the generated bindings in this repository and instead document how to use wit-bindgen-rust and the import! macro to write code that uses the wasi-nn interface.

[abrown]

cc: @brianjjones