Should be doing regularised _2F1

_2F1 sucks compared to regularised _2F1 (bold F in https://dlmf.nist.gov/15.2 or `Hypergeometric2F1Regularized` in mathematica), as the regularised variant avoids the artificial polls at negative integers for `c`. We really should compute the regularised version and then define
```julia
_₂F₁(a, b, c, z) = gamma(c)*regularized_₂F₁(a, b, c, z)
```