add method "free"

src/runtime/c/pgf/db.h

@@ -77,6 +77,11 @@ public:
         return current_db->malloc_internal(bytes);
     }
 
+    template<class A>
+    static void free(ref<A> o) {
+        return current_db->free_internal(o.as_object());
+    }
+
     template<class A>
     static ref<A> get_root() {
         return current_db->get_root_internal();