Remove async func support.

[sunfishcode]

The async keyword has been removed from the async proposal, in favor
of using plain functions that with future or stream types. See also
the component-model PR to remove the syntax.

This removes the (now) old async func suppport from wit-bindgen, to make
it easier to develop the current Canonical ABI proposal.

[sunfishcode]

Unfortunately, the wit-bindgen demo depends on async infrastructure, so I've disabled running the demo in CI for now.

[alexcrichton]

I agree that this is the best way forward. I think it's best to get wit-bindgen into alignment with the currently-written spec and then subsequently re-add features like async as it fits within the current iteration of the component model. Otherwise trying to adapt everything here to the new model seems onerous. I do think that a good bit of what's removed here is going to come back, but it seems so likely to come back in a different form that it's not worth preserving outside of the git history.

It might be worth leaving this open for a day or two to ensure no one else has any objections, but otherwise seems reasonable to me to merge 👍

[alexcrichton]

Unfortunately, the wit-bindgen demo depends on async infrastructure, so I've disabled running the demo in CI for now.

The error I see in CI is:

../../static/main.ts(85,19): error TS2339: Property 'setWasmtimeAsync' does not exist on type 'Config'.

which I think should be easy to fix? I don't believe the demo use actual async function anywhere, that's just a config option which configures what the generated code looks like.

[sunfishcode]

Ah, ok. I've now re-enabled the demo in CI, and just removed the async checkbox, which I think makes sense looking forward, as async will be implied by types rather than being a mode.