implemented search accelerator

implemented search accelerator

greatly improves speed compared to non-accelerated,
especially for slower files.

On my laptop, -b12 :
```
calgary.tar :  4.3 MB/s =>  9.0 MB/s
enwik7      : 10.2 MB/s => 13.3 MB/s
silesia.tar :  4.0 MB/s =>  8.7 MB/s
```

Note : this is the simplified version,
without handling dictionaries, external buffer, nor pattern analyzer.
Current `dev` branch on these samples gives :
```
calgary.tar :  4.2 MB/s
enwik7      :  9.7 MB/s
silesia.tar :  3.5 MB/s
```

interestingly, it's slower,
presumably due to handling of dictionaries.