move ntt classes into a package of their own
fix: division in GF(257)
add byte -> gf(256/257) converter

We want to do some math in GF257, alas to make this work we'll have
to encode values 0..256 (larger than 2^8) into something fitting byte
values. This is a simple encoder which encodes 255/256 as [255, 0/1].

Adopt Shamir and Rabin to work with this new encoder