digamma special function: merge shared code.
digamma special function: merge shared code.

Moved type-specific code into a helper class digamma_impl_maybe_poly<Scalar>.

ritsu1228/eigen,ritsu1228/eigen,pasuka/eigen,ROCmSoftwarePlatform/hipeigen,pasuka/eigen,ROCmSoftwarePlatform/hipeigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,pasuka/eigen,ritsu1228/eigen,ritsu1228/eigen,pasuka/eigen,pasuka/eigen,ROCmSoftwarePlatform/hipeigen