Adding avx512_vnni inline + copy elision

Adding avx512_vnni inline + copy elision

Interesting revelation while benchmarking all of this is that our
chunkmemset_avx seems to be slower in a lot of use cases than
chunkmemset_sse.  That will be an interesting function to attempt to
optimize.

Right now though, we're basically beating google for all PNG decode and
encode benchmarks.  There are some variations of flags that can
basically have us trading blows, but we're about as much as 14% faster
than chromium's zlib patches.

While we're here, add a more direct benchmark of the folded copy method
versus the explicit copy + checksum.