fix: berlekamp-welch with gf(257) and optimizations

* again this was a gf(2^n)-ism: add ain't the same as sub within
  gf(257)
* removed some (by now) unneeded assertions
* added better unit tests for gf-arithmentic (to allow for removed
  assertions)
* PolyHash should check that input has the right format
* pre-compute some exp/pows for gf(257)