Provide a higher-level public API for Rust users

The functions defined in `wasi_nn::wasi_ephemeral_nn::*` are the raw Wasm symbols used to link with the host's implementation of wasi-nn. The `wasi-nn` Rust bindings should not expose these, since they should be providing a higher level of abstraction. These symbols are visible in the documentation [here](https://docs.rs/wasi-nn/0.1.0/wasi_nn/wasi_ephemeral_nn/index.html).