Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/cxx_string.rs

@@ -0,0 +1,15 @@
+use cxx::{let_cxx_string, CxxString};
+
+#[test]
+fn test_async_cxx_string() {
+    async fn f() {
+        let_cxx_string!(s = "...");
+
+        async fn g(_: &CxxString) {}
+        g(&s).await;
+    }
+
+    // https://github.com/dtolnay/cxx/issues/693
+    fn assert_send(_: impl Send) {}
+    assert_send(f());
+}