nmod_mat_solve_tril_classical, nmod_mat_solve_triu_classical

Forward, respectively back, substitution, for an $r \times r$ triangular matrix. If $B$ has at least $\max(4, r/4)$ columns ($\max(4, r/2)$ for $n > 2^{31}$, or $n > 2^{15}$ with 32-bit words), the rows of $X$ are computed one after the other, each as the row of $B$ minus a linear combination of the rows already computed, with delayed reductions: the products are accumulated by vectors in one, two or three words and each entry is reduced once (or a few times, for some moduli below $2^{32}$). Otherwise each column of $X$ is computed entry by entry, each entry with one dot product and one reduction, which is faster with few long columns. The inverses of the diagonal entries are computed once.