zlib 1.2.3 -> zlib 1.2.8

    * Update bundled zlib version to 1.2.8 from 1.2.3. Changes of note:
    "Wholesale replacement of gz* functions with faster versions"
    "Added LFS (Large File Summit) support for 64-bit file offsets"
    "Fix serious but very rare decompression bug"

Closes #335 

Ready for reviewing