Another productive day

Today I did the following:

• implement hensel lifting code for fmpzxx
• fix a const correctness problem in fmpz_poly_mat
• implement prod for fmpz_poly_matxx
• implement solve_fflu_precomp for fmpz_poly_matxx and nmod_poly_matxx
• implement various four-argument functions for nmod_polyxx
• implement bit_pack and bit_unpack for fmpzxx

 This means that the only coding related tasks of priority (H) or (M) left on my TODO list are ustream and ?adic*. Yay!