Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/unique_ptr.rs

@@ -0,0 +1,10 @@
+#![allow(clippy::let_underscore_drop)]
+
+use cxx::{CxxString, UniquePtr};
+
+#[test]
+#[should_panic = "called deref on a null UniquePtr<CxxString>"]
+fn test_deref_null() {
+    let unique_ptr = UniquePtr::<CxxString>::null();
+    let _: &CxxString = &unique_ptr;
+}