Prepare optimized and specialized variants of AlphabetEncoding for each input sequence type

[slowikj]

1. AlphabetEncoding as a template instead of a class
2. Move the responsibility of checking if a sequence element is allowed to AlphabetEncoding from the algorithm
   (for encode method: return a notAllowedElementCode if the input element is invalid)
3. Each sequence type has custom specialization of AlphabetEncoding