TST: special/mpmath: the digamma failure was just too low dps
BUG: special: add an asymptotic expansion to zeta

This avoids underflows in zeta(s, a) for large a and not too large s
BUG: special: fix loss of precision and wrong branch in expi()
BUG: special: improve complex-valued exp1 accuracy

Use a more sensible method for continued fraction evaluation,
and adjust the region in which the power series is used.

The accuracy is around 1e-12.
Merge pull request #2486 from pv/special-digamma-zeta-expi

BUG: special: scrub corner cases in zeta, exp1