modular_matrix: don't use STL iterators in {mul, sub}_col and friends.

modular_matrix: don't use STL iterators in {mul, sub}_col and friends.

Not using STL iterators makes the code simpler and cleaner. Also we don't
have to argue whether the standard ([lib.random.access.iterators]) imposes
any pre/post-condictions on operator+=.
(cherry picked from commit 59ec13895c97ffd74979a0b811a7571f6de56386)