start with NTT-based encoder/decoder
work on gf(257) / ntt

* add more test cases for ntt
* implement ntt-based shamir-encoder/decoder within a unit test,
  remove premature NTTEncoder
* fix gf(257) pow(a,n), it was creating results for large n

add additional test case

add more tests

fix: a^n in gf(257)

make NTT-Shamir TestCase work