Explore re-building on top of WITX2 and `witx-bindgen`

[radu-matei]

ref https://github.com/bytecodealliance/witx-bindgen

This is a long-term issue, as we will lose support for AssemblyScript (and other potential language bindings provided by https://github.com/jedisct1/witx-codegen, so we have to wait either for witx-codegen or witx-bindgen to get AssemblyScript support), but I think it is worth rewriting the interface and generating both Wasmtime and Rust bindings.