beta(-n,n) gives NaN, when n is an integer

[QMrpy]

Beta function at negative integer argument x=n should give ((-1)^n)/n, but instead it gives NaN.The evaluation at negative integers must be done by taking limits close to -n, where the pole at negative integers cancels out

[Peiffap]

Some work on this was done in JuliaLang/julia#14349.

[simonbyrne]

See also discussion in JuliaLang/julia#14256. Would be also good to address cancellation issues when arguments have large difference in magnitudes (see JuliaLang/julia#4301 (comment)).

Several utility functions were added in #165 which might be reused for this.