Wasm multi-value returns support

[greenhat]

Look into its support in Rust->LLVM->Wasm as well as in Wasm CM and its tooling.

[greenhat]

@bitwalker The rustc support is in place but there are still some issues (see 1, 2)

There is an interest to enable it in LLVM by default this year but it might be a bumpy ride - see 1

Canonical ABI does not support milti-value returns yet (see docs and code)

EDIT: "The knobs are there" but there are no plans to implement it. See the answer in the comment below.

[greenhat]

I got reply at Wasm zulip chat https://bytecodealliance.zulipchat.com/#narrow/stream/206238-general/topic/.E2.9C.94.20Multi-value.20returns.20support.20in.20Wasm.20component.20model.20in.20.2E.2E.2E/near/418666262

Currently AFAIK there's no plans to implement it. The knobs are all there to take advantage of it and we have binary encodings in place to eventually take advantage of it in the future too. For now it's blocked on there's no way to write a source-level program (e.g. C or Rust) which uses multi-value returns in a stable fashion.