wit, wit/bindgen: fix handling of transitively included @since gates across package boundaries

[ydnar]

• wit: add async to WIT keywords list
• wit: sort unversioned packages first
• wit: add Stability.Versioned method
• wit: skip incompatible @since gates from transitive included worlds from different packages
• testdata/codegen: update codegen JSON
• testdata: update fixtures for wit/bindgen: flattened worlds with versioned feature gates cannot be correctly serialized  #306 and @since gates should not transitively apply across package boundaries when worlds are included #325, add fixture from wit-parser crate

[vados-cosmonic]

Hey @ydnar just to pop in here and keep you up to date, error-contexts are going to be removed from implementations for stream/future but will continue to exist (and be evolved down the road) for general use:

WebAssembly/component-model#477
(barring of course some intense breakdown of consensus)

Hopefully this eases implementation that you're already working on! We also have some WAST tests and Rust tests w/ components that might be useful for reference.

[ydnar]

Can we rename it to just error?

[vados-cosmonic]

What immediately comes to mind is that error would become a reserved name and then %error would be required in many places and up and breaking WITs... The other thing that might stand in the way is that the error-context is not actually the error (and won't be for some time -- if ever), it's more information related to the error than anything else (though I would personally hope we can pass around data in there somehow, someday).

I think it's certainly worth discussing in that thread though -- I don't know exactly why it wasn't called error in the first place, but I assume there are good reasons.