fn rstrip(_: &mut VM, self_val: Value, args: &Args2) -> VMResult {
    args.check_args_num(0)?;
    let string = self_val.as_string().unwrap();
    let trim: &[char] = &[' ', '\n', '\t', '\x0d', '\x0c', '\x0b', '\x00'];
    let res = string.trim_end_matches(trim);
    Ok(Value::string(res))
}
String#rstrip
https://docs.ruby-lang.org/ja/latest/method/String/i/rstrip.html
ruruby/src/builtin/string.rs