Formal series by formula

Earlier FPS design using generator didn't work.
Here is other using formula based on #7618
> Now with formula, ideally gen should be of form f(k)*x**k.

or just f(k), or a tuple of pairs (f(i), k+i), where f(i) is a function for "k+i"-th coefficient.  You should decide how you want to represet the power series.  And then - you should keep the form of this representation while doing arithmetics and so on.

> 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

It may be possible with something like Picewise.  But...