fn extend(vm: &mut VM, self_val: Value, _: &Args2) -> VMResult {
    let mut singleton = self_val.get_singleton_class()?;
    for arg in vm.args().to_owned() {
        let module = arg.expect_module("arg")?;
        singleton.append_include(&mut vm.globals, module);
    }
    Ok(self_val)
}
Object#extend(*modules) -> self
https://docs.ruby-lang.org/ja/latest/method/Object/i/extend.html
ruruby/src/builtin/object.rs