Remove witx crate dependency (or maintain it)

#### Feature

I think it makes sense to either work around the witx crate or take maintenance over it. (Not sure how crucial it is so cannot judge which is the better approach.)

The witx crate is quite old and has some (very) old dependencies:
- thiserror 1 (probably OK, but for my project it is now the only one that uses version 1)
- wast 35 (latest version is 248)

#### Benefit

No need to compile and depend on these old dependencies. Reduces transitive-dependency hell a bit.

#### Implementation

Easiest is probably to start to maintain it. Looking at https://crates.io/crates/witx/reverse_dependencies?page=1 it seems like wiggle-* and wasmtime-* are the main users.

The code is available at https://github.com/WebAssembly/WASI/tree/wasi-0.1/tools/witx (took me a while to find it)

#### Alternatives

No idea.