Breaking change to rref over a ring

[Bill Hart]

Hi all,

We will be changing the order of arguments returned by rref over a
ring. In fact, Nemo already has the correct order r, A, d, but
AbstractAlgebra (for generic matrices) currently has the order r, d,
A, and this will be changed to match Nemo.

It's unlikely that anyone is actually using this function, but we
thought it best to communicate the change.

Note that rref over a field will not be affected as it only returns
two arguments in the first place.

Bill.