adding an efficient Pochhammer

There are many special functions which use the [Pochhammer](https://en.wikipedia.org/wiki/Falling_and_rising_factorials) function, and I believe this package could use and efficient version via the `@horner` macro.

Is that within the scope for a PR?

It is also worth considering including the Stirling numbers of the first and second kind (though they are already defined in Combinatorics.jl) so as to make implmentations of many other special functions easier -- for example, the [Bell polynomials](http://mathworld.wolfram.com/BellPolynomial.html).