BUG: special: fix infinite runtime in hyp2f1
BUG: special: avoid loss of precision in gegenbauer
BUG: special: fix negative order chebyshev polynomials
TST: special: enable mpmath orthopoly tests
BUG: special: fix negative order jacobi polynomials
TST: special: enable jacobi test
TST: special: fix laguerre polynomial test
BUG: special: fix negative order legendre polynomials
TST: special: fix gegenbauer + legendre tests
BUG: special: fix loss of precision in eval_gegenbauer (int-order)

Use power series instead of recurrence for small x.
BUG: special: fix loss of precision in eval_legendre (int-order)
TST: special/mpmath: adjust parameter ranges probed
TST: special/mpmath: check the small-x expansion regime for gegenbauer and legendre (int-order)
TST: special/mpmath: adjust bei/ber ranges
TST: special/mpmath: add explanations for the redefined functions
Merge pull request #2475 from pv/special-orthopoly

BUG: special: fix up a number of corner cases in orthogonal polynomials