Add compression mode that is better than default.

Approximately 50% of the speed with overall improvement. Roughly the same as zstd level 7.

Some comparisons (best of 3 runs):

```
file	out	level	insize	outsize	millis	mb/s
enwik9	zskp	2	1000000000	317517832	7452	127.96
enwik9	zskp	3	1000000000	298846117	14109	67.59

nyc-taxi-data-10M.csv	zskp	2	3325605752	595200765	14299	221.80
nyc-taxi-data-10M.csv	zskp	3	3325605752	560393969	25808	122.89

github-june-2days-2019.json	zskp	2	6273951764	640407088	14517	412.15
github-june-2days-2019.json	zskp	3	6273951764	628859544	36950	161.93

rawstudio-mint14.tar	zskp	2	8558382592	3379874208	42571	191.72
rawstudio-mint14.tar	zskp	3	8558382592	3301556598	93862	86.96
```