Formal series by formula

Earlier FPS design using generator didn't work.
Here is other using formula based on #7618
More bugs:
```
In [3]: from sympy.series.formal import findgen

In [4]: f1=findgen(sin(x), x)

In [5]: f1(k)
Out[5]: 
⎡⎛              k                    ⎞⎤
⎢⎜          -1/4                     ⎟⎥
⎢⎜──────────────────────────, 2⋅k + 1⎟⎥
⎣⎝RisingFactorial(3/2, k)⋅k!         ⎠⎦

In [6]: f2=findgen(100500*sin(x), x)

In [7]: f2(k)
Out[7]: 
⎡⎛              k                    ⎞⎤
⎢⎜          -1/4                     ⎟⎥
⎢⎜──────────────────────────, 2⋅k + 1⎟⎥
⎣⎝RisingFactorial(3/2, k)⋅k!         ⎠⎦

```