Remove async features stream and future for Preview 2

[esoterra]

If I remember correctly and it hasn't changed, wasi-snapshot-preview2 isn't going to rely on async features, but this repo still uses stream and future in its .wit.

[sunfishcode]

Yes. I'm expecting to do that once wit-bindgen supports use and we can start keeping the prototype world in separate files which can be more easily coordinated with the upstream proposal repos.

[esoterra]

That makes sense, just wanted to make sure it was on your radar

[sunfishcode]

WebAssembly/wasi-proposal-template#2 updates the wit-bindgen; if that's accepted, the next step is to start updating all the proposal repos.

[rektide]

not sure what impact this has on the ticket or what happens next with wasi-filesystem, but stream and future have landed in the component model.
WebAssembly/component-model#405

[sunfishcode]

Indeed, and we're excited to adopt it. Some of the WASI proposals have added wit-0.3.0-draft directories showing what they'll look like with the new async features. I don't know if anyone's working on that for wasi-filesystem yet, but that would be one of the next steps.