pub(crate) fn define_method(&mut self, target_obj: Value, id: IdentId, method: FnId) {
        self.globals
            .get_class_if_object(target_obj)
            .add_method(&mut self.globals, id, method);
    }
Define a method on `target_obj`.
If `target_obj` is not Class, use Class of it.
ruruby/src/vm/executor.rs