several improvements to rref_multi_mod, changed to use the new _nmod_mat_rref too
several improvements to rref_multi_mod, changed to use the new _nmod_mat_rref too

fredrik-johansson/flint2,jpflori/flint2,jpflori/flint2,fredrik-johansson/flint2,jpflori/flint2,wbhart/flint2,dsroche/flint2,dsroche/flint2,dsroche/flint2,dsroche/flint2,wbhart/flint2,fredrik-johansson/flint2,wbhart/flint2,jpflori/flint2