_flint_mpn_poly_mulmid_classical

Sets $(\mathit{res}, (\mathit{nhi} - \mathit{nlo}) s)$ to the coefficients $[\mathit{nlo}, \mathit{nhi})$ of the product of $(f, \mathit{flen}\, n_1)$ and $(g, \mathit{glen}\, n_2)$, viewed as polynomials with $n_1$-limb respectively $n_2$-limb coefficients, computed modulo $B^s$ by the classical algorithm. Requires $n_1 \ge n_2$ and $s \in \{n_1 + n_2 - 1, n_1 + n_2, n_1 + n_2 + 1\}$. The coefficients are unsigned for sgn = 0, two's complement for sgn = 1, and sign-magnitude for sgn = 2 (a sign limb followed by $n_1$ respectively $n_2$ magnitude limbs; the output is two's complement). The results are exact provided that the requested coefficients fit in $s$ limbs. Squaring is detected when f == g and flen == glen.