Fix yhirose#60

Author: yhirose-jw

peglib.h

@@ -3010,7 +3010,7 @@ struct AstBase : public Annotation
     const std::string                 token;
 
     std::vector<std::shared_ptr<AstBase<Annotation>>> nodes;
-    std::shared_ptr<AstBase<Annotation>>              parent;
+    std::weak_ptr<AstBase<Annotation>>                parent;
 };
 
 template <typename T>