Formal series by formula

Earlier FPS design using generator didn't work.
Here is other using formula based on #7618
More tests (see [2], sec 9 "Some Results"):

9.1 - doesn't work
```
In [2]: f2=findgen((asin(x)/x)**2, x)

In [3]: f2(k)
Out[3]: []
```

9.2 - slightly better, but doesn't work too:
```
In [6]: f4=findgen(exp(asinh(x)), x)

In [7]: f4(k)
Out[7]:
⎡⎛    k                              ⎞  ⎛    k                               ⎞⎤
⎢⎜(-1) ⋅RisingFactorial(-1/2, k)     ⎟  ⎜(-1) ⋅RisingFactorial(0, k)         ⎟⎥
⎢⎜──────────────────────────────, 2⋅k⎟, ⎜───────────────────────────, 2⋅k + 1⎟⎥
⎣⎝              k!                   ⎠  ⎝  RisingFactorial(3/2, k)           ⎠⎦

In [25]: [(__[0].simplify(),__[1].simplify()) for __ in f4(k)]
Out[25]:
       k
  -(-1) *(k - 3/2)!
[(------------------, 2*k), (0, 2*k + 1)]
         ____
     2*\/ pi *k!
```

9.4 - hangs.

On the next page - last two idenities are broken:
```
In [42]: f6=findgen(exp(x) - 2*exp(-x/2)*cos(pi/3+sqrt(3)*x/2), x)
         
In [43]: f6(k)
Out[43]: 
                           -k
                         27
[(--------------------------------------------------, 3*k + 1)]
  RisingFactorial(2/3, k)*RisingFactorial(4/3, k)*k!
```
(wrong coefficient)

```
In [53]: f7=findgen(log((1+x)/(1-x))/2-atan(x), x)
         
In [54]: f7(k)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
...
```