Add LIT_MEM define to use more memory for a small deflate speedup.

Add LIT_MEM define to use more memory for a small deflate speedup.

A bug fix in zlib 1.2.12 resulted in a slight slowdown (1-2%) of
deflate. This commit provides the option to #define LIT_MEM, which
uses more memory to reverse most of that slowdown. The memory for
the pending buffer and symbol buffers is increased by 25%, which
increases the total memory usage with the default parameters by
about 6%.

madler/zlib#ac8f12c97d1afd9bafa9c710f827d40a407d3266