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

Fix bugs in orthogonal polynomials: (i) correct definition for negative-integer-order polynomials, (ii) fix up loss of precision in gegenbauer and hermite. The corresponding comparisons to mpmath now pass.

This doesn't touch the non-integer order functions --- those fall back to hypergeometric functions, which should be checked too later on.