This changes how matching is done in deflate algorithm.
The major change is that we do not look for matches that are only
3 bytes in length, matches must be 4 bytes at least.
Contrary to what you would expect this actually improves the
compresion ratio, since 3 literal bytes will often be shorter
than a match after huffman encoding.
This varies a bit by source, but is most often the case when the
source is "easy" to compress.
Second of all, a "stronger" hash is used. The hash is similar to
the hashing function used by Snappy.
Overall, the speed impact is biggest on higher compression levels.
I intend to replace the "speed" compression level, which can be
seen in CL 21021.
The built-in benchmark using "digits" is slower at level 1.
I see this as an exception, since "digits" is a special type
of data, where you have low entropy (numbers 0->9), but no
significant matches. Again, CL 20021 fixes that case.
NewWriterDict is also made considerably faster, by not running data
through the entire encoder. This is not reflected by the benchmark.
Overall, the speed impact is biggest on higher compression levels.
I intend to replace the "speed" compression level.