fn even(_: &mut VM, self_val: Value, args: &Args2) -> VMResult {
    args.check_args_num(0)?;
    if let Some(i) = self_val.as_fixnum() {
        Ok(Value::bool(i.is_even()))
    } else if let Some(b) = self_val.as_bignum() {
        Ok(Value::bool(b.is_even()))
    } else {
        unreachable!()
    }
}
even? -> bool

https://docs.ruby-lang.org/ja/latest/method/Integer/i/even=3f.html
ruruby/src/builtin/integer.rs