Better Arithmetic code abstraction

### Arithmetic
* new binary probability primitive (```BinaryModel```)
* ```cut_threshold``` mechanics removed in favor of an automatic trigger
* helper Decoder/Encoder are introduced for more than simple use cases
* better IoError passing for the Reader/Writer implementations

### BWT
* a counter overflow fixed for ```Radix```

### DC
* decoder is now aware of the full-alphabet inputs, doesn't require initial order now
* decoder is now using a fetch closure instead of distances slice