FEC NF4: update w/ context
FEC NF4: update w/ context

- init r_powers and update decoding 2 steps
- init fft_2k
- re-implement decode_init() to re-implement context->init() since it
needs to compute derivative of a polynomial. This operation is different
for NF4 as exponent of a polynomial is a tuple of GF symbols.

vrancurel/ntl,vrancurel/ntl,vrancurel/ntl