Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/opaque_autotraits.rs

@@ -0,0 +1,16 @@
+#[cxx::bridge]
+mod ffi {
+    extern "C++" {
+        type Opaque;
+    }
+}
+
+fn assert_send<T: Send>() {}
+fn assert_sync<T: Sync>() {}
+fn assert_unpin<T: Unpin>() {}
+
+fn main() {
+    assert_send::<ffi::Opaque>();
+    assert_sync::<ffi::Opaque>();
+    assert_unpin::<ffi::Opaque>();
+}