Faster bit shifting on u32 when shift amount is statically known

[Sword-Smith]

We want to speed up this code which is used in NTT

        fn bitreverse(mut n: u32, l: u32) -> u32 {
            let mut r: u32 = 0;
            let mut i: u32 = 0;
            while i < l {
                r = (r << 1) | (n & 1);
                n >>= 1;
                i += 1;
            }

            return r;
        }