Improve root calculation for KZG setup()

[chiro-hiro]

Hi guys, I'm learning KZG so I checked halo2 source code. Found some minor issues and can be improved.

                let mut root = <Bn256 as Engine>::Scalar::ROOT_OF_UNITY_INV
                    .invert()
                    .unwrap();
                for _ in k..<Bn256 as Engine>::Scalar::S {
                    root = root.square();
                }

Is equivalent to

                let root = <Bn256 as Engine>::Scalar::ROOT_OF_UNITY
                    .pow_vartime([1 << (<Bn256 as Engine>::Scalar::S - k)]);

Result:

                        root-of-unity time:   [1.4413 µs 1.4443 µs 1.4473 µs]                           
                        change: [-89.921% -89.897% -89.873%] (p = 0.00 < 0.05)
                        Performance has improved.

[chiro-hiro]

@CPerezz Could you please check this?

[han0110]

Thanks for catching this!