Faster East Slavic pluralization

I was investigating how Russian pluralization works and noticed that current code creates ranges and converts it to array on each invocation which is suboptimal behavior. I'd much faster if it just used pre-cached sets with O(1) lookup time. So, here goes pull-request implementing this change achieving 2.5x performance increase.

Benchmark results:

```
            old code        78 i/100ms
            new code       193 i/100ms
-------------------------------------------------
            old code      790.3 (±3.0%) i/s -       3978 in   5.038320s
            new code     1919.9 (±2.9%) i/s -       9650 in   5.030570s
```

Benchmark code can be found [here](https://gist.github.com/rwz/8246313).