fn dump(_: &mut VM, self_val: Value, args: &Args2) -> VMResult {
    args.check_args_num(0)?;
    //let self_ = self_val.as_string().unwrap();
    let str: String = format!("{:?}", self_val);
    Ok(Value::string(str))
}
String#dump
https://docs.ruby-lang.org/ja/latest/method/String/i/dump.html
ruruby/src/builtin/string.rs