s2: Add 'best' compression option

[klauspost]

Add compression mode that will compress the data very well, but at a significant performance decrease.

Mainly for offline compression, but where decompression speed should still be high and compatible with other S2 compressed data.

Comparing on 16 core CPU:

* enwik10:
Default... 10000000000 -> 4761467548 [47.61%]; 1.098s, 8685.6MB/s
Better... 10000000000 -> 4225922984 [42.26%]; 2.817s, 3385.4MB/s
Best... 10000000000 -> 3667646858 [36.68%]; 35.995s, 264.9MB/s

* github-june-2days-2019.json:
Default... 6273951764 -> 1043196283 [16.63%]; 431ms, 13882.3MB/s
Better... 6273951764 -> 950079555 [15.14%]; 736ms, 8129.5MB/s
Best... 6273951764 -> 846260870 [13.49%]; 8.125s, 736.4MB/s

* nyc-taxi-data-10M.csv:
Default... 3325605752 -> 1095998837 [32.96%]; 324ms, 9788.7MB/s
Better... 3325605752 -> 960330423 [28.88%]; 602ms, 5268.4MB/s
Best... 3325605752 -> 794873295 [23.90%]; 6.619s, 479.1MB/s

* 10gb.tar
Default... 10065157632 -> 5916578242 [58.78%]; 1.028s, 9337.4MB/s
Better... 10065157632 -> 5650133605 [56.14%]; 2.172s, 4419.4MB/s
Best... 10065157632 -> 5246578570 [52.13%]; 25.696s, 373.6MB/s

* consensus.db.10gb:
Default... 10737418240 -> 4562648848 [42.49%]; 882ms, 11610.0MB/s
Better... 10737418240 -> 4542443833 [42.30%]; 3.3s, 3103.5MB/s
Best... 10737418240 -> 4272335558 [39.79%]; 38.955s, 262.9MB/s