[math-fun] Hilbert curves, Lunnon, fxt

It looks like Lunnon did a comparatively good job re multi-D Hilbert curves on Arndt's web page. However, I still think there ought to be code much better (meaning simpler & shorter) that nobody has written yet. I don't think it is terribly hard, but it is a pain to figure out the details & get them right, and so far that pain barrier has proved sufficient to stop the code from being.