Popcnt vectorization

Hi Simon,

Same as in the other pull request, added the missing masks

Best regards,
Diego
Please define the methods further up:

```c++
#ifdef __AVX2__
inline uint64_t cnt256(__m256i x);
#endif
```

same for cnt128:

```c++
#ifdef __sse4_2__
    uint64_t cnt128(__m128i x);
#endif 
```