KronMatrix stores the two factor matrices A (m x n)
and B (p x q) without forming the full (mp x nq) Kronecker
product. Matrix-vector and matrix-matrix products are evaluated
using the vec-permutation identity
(A x B) vec(X) = vec(B X t(A)), keeping memory use at
O(mn + pq) rather than O(mnpq).
Slots
ANumeric matrix; the left factor of the Kronecker product.
BNumeric matrix; the right factor of the Kronecker product.