Add support for @deprecated gates to WIT

[yoshuawuyts]

Closes #374. Adds support for @deprecated gates to WIT. While the text in this PR is ready for review now, we should hold off on merging this until we have implementation feedback. Thanks!

r? @lukewagner

[lukewagner]

lgtm! Let me know once there's any implementation to validate this and then we can merge.

[alexcrichton]

I've got an initial implementation of this at bytecodealliance/wasm-tools#1687 with the most notable difference being my prior comment where @deprecated is required to be paired with @unstable or @stable. Easy to change, but figured I'd put a stick in the sand somewhere.

[yoshuawuyts]

@alexcrichton thanks for putting that PR up; that's great! Also it sounds good to me to require @deprecated to always be paired with another gate.

Is there a use case when people might want to combine @unstable and @deprecated? The way I was thinking about this is that in order to mark something as @deprecated it must first be marked as stable. And by definition anything marked with @unstable is not stable?

(Also I wasn't sure what you meant by "stick in the sand" here - I assumed something like: "starting point" and not like "line in the sand"? I had trouble looking it up 😅)

[alexcrichton]

My thinking for @unstable and @deprecated is that in my experience in Rust if something stays as @unstable for too long but it's widely used then it's defacto-stable and a transition plan of sorts is needed for the eventual replacement. IIRC there's been #[deprecated] and #[unstable] APIs in Rust, and this was intended to mirror that. It's definitely a bit of a niche use case for sure.

Also I wasn't sure what you meant by "stick in the sand" here

Oh that's just me mixing up idioms and producing nonsense, you can ignore me.

[yoshuawuyts]

Okay, great - that's a good enough motivation for me. I think that means that once we've merged bytecodealliance/wasm-tools#1687 we should be ready to merge this too!

edit: modulo removing message from this proposal - will do that now!