fn instance_exec(vm: &mut VM, self_val: Value, args: &Args2) -> VMResult {
    let block = args.expect_block()?;
    let res = vm.eval_block_self(block, self_val, &args.into(vm));
    res
}
instance_exec(*args) {|*vars| ... } -> object
https://docs.ruby-lang.org/ja/latest/method/BasicObject/i/instance_exec.html
ruruby/src/builtin/basicobject.rs