fn at(vm: &mut VM, self_val: Value, args: &Args2) -> VMResult {
    args.check_args_num(1)?;
    self_val.into_array().get_elem1(vm[0])
}
## instance method Array#[]
- at(nth) -> object | nil

https://docs.ruby-lang.org/ja/latest/method/Array/i/=5b=5d.html
ruruby/src/builtin/array.rs