Zlib replacement with optimizations for next generation systems.

The motivation for this fork came after seeing several 3rd party contributions
containing new optimizations not getting implemented into the official zlib
repository.

Mark Adler has been maintaining zlib for a very long time, and he has done a
great job and hopefully he will continue for a long time yet.

The idea of zlib-ng is not to replace zlib, but to co-exist as a drop-in
replacement with a lower threshold for code change.