Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/derive_noncopy.rs

@@ -0,0 +1,13 @@
+#[cxx::bridge]
+mod ffi {
+    #[derive(Copy)]
+    struct TryCopy {
+        other: Other,
+    }
+
+    struct Other {
+        x: usize,
+    }
+}
+
+fn main() {}