Do Horner's rule on polynomial Hypergeometric functions.

If numeratorial parameters in hypers are nonpositive integers,
* The sums are finite polynomials
* Which can be summed by Horners rule

Start by locating the hypergeometric-function implementations and the handling of nonpositive integer numerator parameters. Confirm the finite-polynomial cases and define the expected Horner evaluation behavior, then add focused coverage for those cases. Done means those finite sums use Horner's rule without changing other hypergeometric results.

Written by the indexing model from the issue text.