fix issue #8323

	harmonic(n).evalf() and catalan(n).evalf()
Seems to me now like the cleaner to overload `.evalf()` rather than define `._eval_evalf()`, but then the documentation would need to be duplicated, and maybe some other things.  And calling `evalf` inside `_eval_evalf` seems to be kicking the bucket down the road.  Instead, I think I'll opt for manually implementing the rewrite calls, and calling `_eval_evalf` there as needed.