Finish std::shared_ptr -> Ref migration

src/reference.hpp

@@ -318,7 +318,7 @@ template <typename T> struct WeakRef {
    * @brief Attempt to promote WeakRef to Ref
    * @return Valid Ref if object still alive, empty Ref otherwise
    */
-  Ref<T> lock() {
+  Ref<T> lock() const {
     if (!control_block) {
       return Ref<T>();
     }