Async bindings generation for C (#1291)

* Extract Rust's parsing of `--async` to a shared library

This should hopefully enable it to share the same parsing of this option
amongst all bindings generators.

* Implement async support in the C bindings generator

Developed through tests in this repository for now.

* Review comments

Author: alexcrichton

.github/workflows/main.yml

@@ -132,10 +132,11 @@ jobs:
         curl -L https://github.com/bytecodealliance/wasip3-prototyping/releases/download/dev/wasmtime-dev-x86_64-linux.tar.xz | tar xJvf -
         echo "WASMTIME=`pwd`/wasmtime-dev-x86_64-linux/wasmtime" >> $GITHUB_ENV
     - run: |
-        cargo run test --languages rust tests/runtime-async \
+        cargo run test --languages rust,c tests/runtime-async \
           --artifacts target/artifacts \
           --rust-wit-bindgen-path ./crates/guest-rust \
           --rust-target wasm32-wasip1 \
+          --c-target wasm32-wasip1 \
           --runner "$WASMTIME -W component-model-async"
 
   test_unit:

Cargo.lock

@@ -23,3 +23,7 @@ wasm-metadata = { workspace = true }
 anyhow = { workspace = true }
 heck = { workspace = true }
 clap = { workspace = true, optional = true }
+indexmap = { workspace = true }
+
+[features]
+clap = ['dep:clap', 'wit-bindgen-core/clap']