fq_mat_mul_KS

Every entry, a polynomial of degree less than $d$ with coefficients in $[0, p)$, is packed into an integer with fq_bit_pack, using fields of $b = \operatorname{bits}((p - 1)^2 k d) + 1$ bits for an inner dimension $k$, enough to hold every coefficient of a sum of $k$ products of two such polynomials. The integer matrices are multiplied with fmpz_mat_mul, which uses multimodular and BLAS-like methods (see Multiplying matrices over Z/nZ), and the entries of the product are unpacked with fq_bit_unpack, which reduces modulo $p$ and the defining polynomial. One reduction per entry is needed, and the product of the packed matrices has entries of about $2db$ bits.