fn next(_: &mut VM, self_val: Value, args: &Args2) -> VMResult {
    args.check_args_num(0)?;
    let self_val = self_val.as_string().unwrap();
    let val = Value::string(str_next(self_val));
    Ok(val)
}
https://docs.ruby-lang.org/ja/latest/class/String.html#I_NEXT
https://github.com/ruby/ruby/blob/11b8bb99e6722253974c73d96ed653f97495e1c5/string.c#L4273
ruruby/src/builtin/string.rs