Speed and compression improvements

[fhanau]

This pull request adds some additional improvements from ECT. The first commit is a follow-up of #111 and makes huffman encoding even faster. The second one improves blocksplitting by only decreasing the lengthscore when the match is short.

[jibsen]

Here are some results on the Silesia compression corpus:

file            original       fhanau
--------------------------------------
dickens        3.673.398    3.669.761*
mozilla       18.317.359   18.314.126*
mr             3.450.243*   3.452.055
nci            2.740.616*   2.740.719
ooffice        2.992.131*   2.992.545
osdb           3.607.235    3.607.233*
reymont        1.692.012    1.691.946*
samba          5.107.606*   5.108.959
sao            5.242.412    5.242.359*
webster       11.518.308   11.516.700*
x-ray          5.744.715*   5.744.863
xml              627.187*     627.231
--------------------------------------
              64.713.222   64.708.497
                1516 sec     1541 sec

It looks like it is an improvement in speed/ratio on some files, and not on others. On average I am getting a small compression gain on the files I tested so far though.

[fhanau]

That's worse than I expected. On my benchmarks(png images and enwik8), it improved both speed and compression.

[fhanau]

With all changes from this pull request, zopfli --i1 is 9% faster(tested on enwik8). d680428 will not influence the compression ratio. Note that the first iteration may be slightly slower.

[fhanau]

15% faster than master.
Changes:

• Faster cost model through inlining the cost model function and converting it to table lookups
• Use std::sort instead of C's quicksort for sorting(Note that you need to compile katajainen.c as C++ for the speed improvement. When compiled as C the old sort is used.)

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored by someone other than the pull request submitter. We need to confirm that they're okay with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.

[parasew]

Are there any updates to this status, or do we have to live with the fact that this will never get merged?