willf/bitset — pull request #19 by salviati (2014-02-16 10:04:33+00:00)
Title: use popcnt if available on amd64

```
benchmark old ns/op new ns/op delta
BenchmarkCount 6837 2605 -61.90%
BenchmarkGetTest 19.4 18.7 -3.61%
BenchmarkSetExpand 3083 3013 -2.27%
BenchmarkSet 21.8 21.4 -1.83%
BenchmarkSparseIterate 33323 33678 +1.07%
BenchmarkIterate 33388 33736 +1.04%
```

(tested on i7 4770S)