Update zlib to the 1.2.12 release
Version 1.2.12 has these key improvements over 1.2.11:
* Fix a deflate bug when using the Z_FIXED strategy that can result in out-of-bound accesses.
* Fix a deflate bug when the window is full in deflate_stored().
* Speed up CRC-32 computations by a factor of 1.5 to 3.
* Use the hardware CRC-32 instruction on ARMv8 processors.
* Speed up crc32_combine() with powers of x tables.
* Add crc32_combine_gen() and crc32_combine_op() for fast combines.
Due to the bug fixes, any installations of 1.2.11 should be replaced with 1.2.12.
Moves all test and documentation files to the location where they are
present in upstream zlib sources. The build files have also been tested
and fixed on Linux. Because they are all similiar, these fixes have
also been mirrored into the macOS and Windows build files too.
