Make fp_complex_promote that complexifies fp_promote.

This is useful for complex returns of real argument.
Just make a thing like what's already hard coded in specfun.h for hankels:
std::complex>

Don't allow complex of complex though.

Start by reading the existing fp_promote_t usage and the hard-coded complex promotion in specfun.h. Trace how real and complex template arguments are handled, then verify that the new fp_complex_promote behavior supports complex returns from real arguments without producing complex-of-complex types.

Written by the indexing model from the issue text.