Update Rust gen to use .wit spec

[danbev]

This commit updates the regeneration script for the Rust bindings to use wasi-nn.wit types instead of wasi-nn.witx.

The scripts has also been run and the bindings (src/generated.rs) have been updated.

This example currently compiles but fails at runtime because the version of the wasi-nn spec that wasmtime is using is not the latest. Once it has been updated this example should work (I've tested it with the version that wasmtime is currently on and it works but that requires changes to the examples in this repository).