Formal series by formula

Earlier FPS design using generator didn't work.
Here is other using formula based on #7618
```
In [2]: s1 = FormalSeries(x, function=sin(x))

In [3]: s2 = FormalSeries(x, function=cos(x))

In [10]: sum1=s1+s2

In [11]: sum1[0]
Out[11]: x + 1

In [12]: sum1[1]
Out[12]: 
   3    2
  x    x 
- ── - ──
  6    2 

```

Is this a feature?

Also, this interface wasn't tested, as most of methods.