`digamma` function

[longemen3000]

the main usecase would be to define AD overloads

[TSGut]

This already exists in SpecialFunctions.jl as Julia native code, so I think we can just port it over ensuring that no dependencies are invoked. At least for Float64 that is. The BigFloat version uses a ccall: https://github.com/JuliaMath/SpecialFunctions.jl/blob/ea68a71c7c3c4053978cb9f8d07980a7b7d8a0b7/src/gamma.jl#L52