Switch old-witx-compat over to the witx crate on crates.io

[Michael-F-Bryan]

This implements the changes mentioned in https://github.com/bytecodealliance/witx-bindgen/issues/85#issuecomment-945938217 for switching witx2's old-witx-compat parser back to using the witx crate from crates.io.

Fixes #85.

[Michael-F-Bryan]

@alexcrichton, how would I handle an old::DeclSyntax::Module(_)? From the source code, it looks like a module is some sort of namespacing feature, but it doesn't look like there is any equivalent in witx2.

I could implement some sort of "Scope" mechanism where entering a module pushes onto the stack and we use that stack whenever we need to convert names, but I have no idea if that makes sense for witx2.

[alexcrichton]

IIRC you can transparently step into a Module since a document contains at most one module, so I think it's basically just peeled? I forget precisely what it corresponds to though...

[alexcrichton]

I'm going to close this since the old-witx-compat feature has since been removed and I don't think we are going to want to add it back soon either.