fn get_elem(vm: &mut VM, self_val: Value, args: &Args2) -> VMResult {
    args.check_args_range(1, 2)?;
    self_val.into_array().get_elem(vm.args())
}
## instance method Array#[]
- self[[nth]] -> object | nil
- self[[range]] -> Array | nil
- self[[start, length]] -> Array | nil

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