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)