Async

WebAssembly is starting a subgroup to talk about stack switching:

https://github.com/WebAssembly/meetings/blob/master/main/2021/CG-03-16.md#agenda-items

https://docs.google.com/presentation/d/1teI1N282B7Dog314G22Emmaur2XCVtqYwwq5c_AK-BE/edit#slide=id.p

Like the rest of WASI, I/O Types are independent of sync/async.