Feature request: Reverse

[stevefan1999-personal]

I'm trying to implement vgf2p8mulb using portable SIMD but I found something intriuging.

            // Accumulate result using bit tests
            let mut result = <$u8_vec_type>::splat(0);
            unroll! {
                for i in 0..8 {
                    let bit = (b >> (i as u8)) & <$u8_vec_type>::splat(1);
                    result ^= bit * a_powers[i];
                }
            }

Although the correct code should be:

            // Accumulate result using bit tests
            let mut result = <$u8_vec_type>::splat(0);
            for i in (0..8).rev() {
                let bit = (b >> (i as u8)) & <$u8_vec_type>::splat(1);
                result ^= bit * a_powers[i];
            }

However, rev cannot be unrolled right now