bigintmat: solveAx to solve Ax=b over any euc.ring where Howell works
 (changed solvexA to solveAx as everything is column oriented)
new interpreter command: solve(A,b) to (pseudo) solve linear equation
Ax=b over any euc ring for bigintmat
allow 0xn nx0 and 0x0 bigintmats (and change some errors accordingly)