Handle un-normal zeros properly in the division-free elimination.

Handle un-normal zeros properly in the division-free elimination.

Call .normal() instead of just .expand(), such that matrix::pivot() finds
the pivot.

Note that this problem also affects matrix::solve() with 'algo'
set to solve_algo::automatic.

Of course, using .normal() makes division_free_elimination only
truly "division-free" if the input matrix didn't have fractions.
In other cases, GCDs will be computed.