fmpz_mpolyl_gcd_brown, fmpz_mpolyl_gcd_brown_threaded_pool

Brown's dense multimodular algorithm [Bro1971] for the GCD and cofactors of nonzero A and B. The integer contents are removed from A and B, which are modified. Images are computed with nmod_mpolyn_gcd_brown_smprime modulo primes p > 2^{\mathrm{FLINT\_BITS}-1} not dividing the GCD \gamma of the leading coefficients, scaled to have leading coefficient \gamma \bmod p and combined by Chinese remaindering, discarding images of too large degree. The loop stops when the product m of the primes exceeds twice \gamma times the larger height of A and B and the reconstructed H, \bar A_H, \bar B_H satisfy 2 \min(\|H\|_\infty \|\bar A_H\|_1, \|H\|_1 \|\bar A_H\|_\infty) < m and likewise for B; this proves H \bar A_H = \gamma A and H \bar B_H = \gamma B without a division. Iv (possibly NULL) supplies degree bounds for the images. The threaded version computes images for different primes in parallel using the given thread pool handles; it is not used by fmpz_mpoly_gcd. Return 0 if the primes run out.