use is_in_rref in rref_mul and change test to compare with fflu rather than general method
use is_in_rref in rref_mul and change test to compare with fflu rather than general method

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