Missing rule for factorial

[mzgubic]

Should be added using @non_differentiable factorial(n::Integer)

[oxinabox]

I considered for a bit that we could define it using the gamma function, tereating it as the embedding of an integer as a float.
But I think that is wrong, since if you perturb the input you will just get an error.
So it should be @non_differentiable
https://math.stackexchange.com/questions/300526/derivative-of-a-factorial