Right now AVX2 implementation looses to C binding in speed.
This is probably, because of 2 things:
1. Go does not inline `mulBitRightx2` in loop iteration.
2. `minmax` is loaded every time from memory.
In this PR:
1. Unroll `mulBitRightx2` manually and use `mulByteRightx2` instead.
2. Generate `minmax` in place without `LOAD/LEA` instructions.
