s2: Add full Snappy output support

[klauspost]

S2 can now fully replace snappy and generate snappy output at all compression levels, including streaming.

Comparison of webdevdata.org-2015-01-07-subset,
53927 files, total input size: 4,014,735,833 bytes. amd64, single goroutine used:

Encoder	Size	MB/s
snappy.Encode	1128706759	725.59
s2.EncodeSnappy	1093823291	899.16
s2.EncodeSnappyBetter	1001158548	578.49
s2.EncodeSnappyBest	944507998	66.00

Comparison of different streams, AMD Ryzen 3950x, 16 cores. Size and throughput:

File	snappy.NewWriter	S2 Snappy	S2 Snappy, Better	S2 Snappy, Best
nyc-taxi-data-10M.csv	1316042016 - 517.54MB/s	1307003093 - 8406.29MB/s	1174534014 - 4984.35MB/s	1115904679 - 177.81MB/s
enwik10	5088294643 - 433.45MB/s	5175840939 - 8454.52MB/s	4560784526 - 4403.10MB/s	4340299103 - 159.71MB/s
10gb.tar	6056946612 - 703.25MB/s	6208571995 - 9035.75MB/s	5741646126 - 2402.08MB/s	5548973895 - 171.17MB/s
github-june-2days-2019.json	1525176492 - 908.11MB/s	1476519054 - 12625.93MB/s	1400547532 - 6163.61MB/s	1321887137 - 200.71MB/s
consensus.db.10gb	5412897703 - 1054.38MB/s	5354073487 - 12634.82MB/s	5335069899 - 2472.23MB/s	5201000954 - 166.32MB/s