start with moving NTTShamir code into business logic

* split up test into better suited test classes
  (esp. the decoders)
* move business logic from unit tests into business
  logic

Still need to make the new NTTShamirPSS class use the
SecretSharing interface.
implement a NTT-based shamir-PSS algorithm

Should work, but is currently very, very slow -- somewhere in the
ballpark of 250x times slower than the normal NTT.