fn rand_(_: &mut VM, _: Value, args: &Args2) -> VMResult {
    args.check_args_num(0)?;
    let num = rand::random();
    Ok(Value::float(num))
}
rand(max = 0) -> Integer | Float
rand(range) -> Integer | Float | nil
https://docs.ruby-lang.org/ja/latest/method/Kernel/m/rand.html
ruruby/src/builtin/kernel.rs