add wasmtime adapter and stdio wrappers

the stdio wrappers will not work on windows, but thats a whole other can
of worms anyway

crates/wasi-c2/build.rs

@@ -3,6 +3,8 @@
 fn main() {
     let cwd = std::env::current_dir().unwrap();
     let wasi = cwd.join("..").join("wasi-common").join("WASI");
+    // this will be available to dependent crates via the DEP_WASI_C2_19_WASI env var:
     println!("cargo:wasi={}", wasi.display());
+    // and available to our own crate as WASI_ROOT:
     println!("cargo:rustc-env=WASI_ROOT={}", wasi.display());
 }