Falling and Rising factorials of integers are integers

Assuming both arguments are integers, the function output is also known to be
integer.

I have also added a test for the output type of `binomial(n, k)`.
I don't think this is true.
```python
>>> rf(1.5, 3)
13.1250000000000
>>> ff(1.5, 3)
-0.375000000000000
>>> ff(-3, -2)
1/2
```