Add AVX512 version of compare256

Add AVX512 version of compare256

Improve the speed of sub-16 byte matches by first using a
128-bit intrinsic, after that use only 512-bit intrinsics.
This requires us to overlap on the last run, but this is cheaper than
processing the tail using a 256-bit and then a 128-bit run.

Change benchmark steps to avoid it hitting chunk boundaries
of one or the other function as much, this gives more fair benchmarks.