Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/enum_receiver.rs

@@ -0,0 +1,11 @@
+#[cxx::bridge]
+mod ffi {
+    enum Enum {
+        Variant,
+    }
+    extern "Rust" {
+        fn f(self: &Enum);
+    }
+}
+
+fn main() {}