pub(crate) fn add_builtin_class_method(
        self,
        globals: &mut Globals,
        name: &str,
        func: BuiltinFunc,
    ) {
        self.get_singleton_class()
            .add_builtin_method_by_str(globals, name, func);
    }
Add BuiltinFunc `func` named `name` to the singleton class of `self`.
ruruby/src/value/class.rs