Update bundled zlib to 1.2.8

Update zlib to 1.2.8, removing the whole `gz*.[ch]` and `compress.c`, `uncompr.c` which we don't care about, and adding another warning for MSVC to ignore.

This involves a couple of merges, as I branched off of the commit which initially added zlib and then merged that into the changes made to that original one, and then merged again to dev to bring it up to date.

This fixes #2343