feat: implement missing async arg result handling

[vados-cosmonic]

This commit implements some (but not all) of the async argument and parameter handling that is emitted for function calls.

[vados-cosmonic]

The Test Async failure seems to be pre-existing (and "allowed to fail") so I'm figuring I can ignore it for now.

I also don't think this PR would have been the one to fix that, given the error, but if so willing to dig in!

Before (on `main`)

------ Failure: cancel-import --------
  runner: tests/runtime-async/async/cancel-import/runner.c
  compiled runner: /home/runner/work/wit-bindgen/wit-bindgen/target/artifacts/cancel-import/runner-c.wasm
  test: tests/runtime-async/async/cancel-import/test.rs
  compiled test: /home/runner/work/wit-bindgen/wit-bindgen/target/artifacts/cancel-import/test-rust.wasm
  error: failed to run `cancel-import`
  
  Caused by:
      command execution failed
      command: "/home/runner/work/wit-bindgen/wit-bindgen/wasmtime-dev-x86_64-linux/wasmtime" "-W" "component-model-async" "/home/runner/work/wit-bindgen/wit-bindgen/target/artifacts/cancel-import/composed-runner.c-test.rs.wasm"
      status: signal: 6 (SIGABRT) (core dumped)
      stderr:
        
        thread 'main' panicked at /home/runner/work/wasip3-prototyping/wasip3-prototyping/crates/wasmtime/src/runtime/component/func/typed.rs:223:67:
        called `Result::unwrap()` on an `Err` value: entry has children
        note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This PR

------ Failure: cancel-import --------
  runner: tests/runtime-async/async/cancel-import/runner.c
  compiled runner: /home/runner/work/wit-bindgen/wit-bindgen/target/artifacts/cancel-import/runner-c.wasm
  test: tests/runtime-async/async/cancel-import/test.rs
  compiled test: /home/runner/work/wit-bindgen/wit-bindgen/target/artifacts/cancel-import/test-rust.wasm
  error: failed to run `cancel-import`
  
  Caused by:
      command execution failed
      command: "/home/runner/work/wit-bindgen/wit-bindgen/wasmtime-dev-x86_64-linux/wasmtime" "-W" "component-model-async" "/home/runner/work/wit-bindgen/wit-bindgen/target/artifacts/cancel-import/composed-runner.c-test.rs.wasm"
      status: signal: 6 (SIGABRT) (core dumped)
      stderr:
        
        thread 'main' panicked at /home/runner/work/wasip3-prototyping/wasip3-prototyping/crates/wasmtime/src/runtime/component/func/typed.rs:223:67:
        called `Result::unwrap()` on an `Err` value: entry has children
        note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[alexcrichton]

Thanks! And yeah I've been meaning to look at that failure but haven't had a chance to get around to it yet