Polygamma functions

[fredrik-johansson]

Extend Stirling series code to compute polygamma functions (i.e. starting the series from some derivative), and optimize for a small number of derivatives by using a direct recurrence instead of binary splitting.

[rickyefarr]

Any updates on the progress of this issue?

[fredrik-johansson]

I plan to work on this, but I have 3-4 major items to do first.

Note that the polygamma function is implemented now, but the remarks about optimizing the digamma function code still apply.