Wrap Rust bindings in higher-level API

As I was working on https://github.com/bytecodealliance/wasmtime/pull/2831, I realized that the wasi-nn Rust bindings are still relatively low level: the example is still forced to [use `unsafe`](https://github.com/bytecodealliance/wasmtime/blob/8387bc0d76896bd724a0db85a28437c5465540b0/crates/wasi-nn/examples/classification-example/src/main.rs#L12) and [jump through memory hoops](https://github.com/bytecodealliance/wasmtime/blob/8387bc0d76896bd724a0db85a28437c5465540b0/crates/wasi-nn/examples/classification-example/src/main.rs#L50-L51) to retrieve the output tensor.