Switch to WIT

The wasi-nn specification has been ported over from the old WITX syntax to the new WIT syntax (see https://github.com/WebAssembly/wasi-nn/pull/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.