Better Arithmetic code abstraction

Arithmetic:
* new binary probability primitive (```FrequencyBinary```)
* ```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