Use https://github.com/klauspost/compress/tree/master/s2#s2-compression
for decompressing Snappy data. I think if this pays off then we could
also try using this in Prometheus for encoding?

Benchmarks show these numbers:
```
benchmark                                                                                       old ns/op     new ns/op     delta
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_500_of_them/OK-16                         1139713       1071864       -5.95%
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_500_of_them/conflict_errors-16            1294286       1207937       -6.67%
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_5000_of_them/OK-16                        11613568      10849901      -6.58%
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_5000_of_them/conflict_errors-16           13100752      12478380      -4.75%
BenchmarkHandlerReceiveHTTP/extremely_large_label_value_10MB,_10_of_them/OK-16                  96128949      89515761      -6.88%
BenchmarkHandlerReceiveHTTP/extremely_large_label_value_10MB,_10_of_them/conflict_errors-16     96769948      90741611      -6.23%

benchmark                                                                                       old allocs     new allocs     delta
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_500_of_them/OK-16                         4578           4578           +0.00%
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_500_of_them/conflict_errors-16            7618           7617           -0.01%
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_5000_of_them/OK-16                        45325          45324          -0.00%
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_5000_of_them/conflict_errors-16           75133          75133          +0.00%
BenchmarkHandlerReceiveHTTP/extremely_large_label_value_10MB,_10_of_them/OK-16                  81             82             +1.23%
BenchmarkHandlerReceiveHTTP/extremely_large_label_value_10MB,_10_of_them/conflict_errors-16     203            202            -0.49%

benchmark                                                                                       old bytes     new bytes     delta
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_500_of_them/OK-16                         1254474       1254280       -0.02%
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_500_of_them/conflict_errors-16            1427791       1427618       -0.01%
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_5000_of_them/OK-16                        13826832      13806752      -0.15%
BenchmarkHandlerReceiveHTTP/typical_labels_under_1KB,_5000_of_them/conflict_errors-16           15412336      15412232      -0.00%
BenchmarkHandlerReceiveHTTP/extremely_large_label_value_10MB,_10_of_them/OK-16                  120041647     120018331     -0.02%
BenchmarkHandlerReceiveHTTP/extremely_large_label_value_10MB,_10_of_them/conflict_errors-16     119704515     119701011     -0.00%
```

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>