Vendor dependencies

Let's see how I like this workflow.

vendor/cxx/tests/ui/generic_enum.rs

@@ -0,0 +1,16 @@
+#[cxx::bridge]
+mod ffi {
+    enum A<T> {
+        Field,
+    }
+
+    enum B<T> where T: Copy {
+        Field,
+    }
+
+    enum C where void: Copy {
+        Field,
+    }
+}
+
+fn main() {}