Add miniz, a drop-in replacement for (parts of) zlib

miniz is a single-file drop-in replacement for most of zlib's
functionality. Having this bundled is quite convenient for us, so we
can rely on compression even on platforms where zlib is not readily
available.

miniz allows disabling unneeded functionality at compile time, by changing
some #defines, so we just compile the parts we really need (namely, the
basic DEFLATE routines).

We may use zlib proper in the future if we find it at compile time, though.