Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/async_fn.rs

@@ -0,0 +1,14 @@
+#[cxx::bridge]
+mod ffi {
+    extern "Rust" {
+        async fn f();
+    }
+
+    extern "C++" {
+        async fn g();
+    }
+}
+
+async fn f() {}
+
+fn main() {}