Formal series by formula

Earlier FPS design using generator didn't work.
Here is other using formula based on #7618
Yes, this is a feature but I see problems with it now that you pointed it.

Earlier with generators, terms used to come in sorted order after adding. So x**2/2 would come before x**3/6.
Now with formula, ideally gen should be of form ``f(k)*x**k``.
But when adding FPS with gen of form ``f(k)*x**k`` and ``f(k)*x**(k+1)``, it is not possible to find FPS of form ``f(k)*x**k``