Implement d2xy, xy2d, d2xyz, xyz2d

[abetusk]

It would be nice to have stand alone conversion functions that convert from spatial dimensions to position along the curve (xy2d, xyz2d) and vice versa (d2xy, d2xyz).

As mentioned in #6, this should be possible in $O( \ln N )$ time (and $O(1)$ ish space).

I'm working on this now (here) and hope to issue a PR in the near future when I'm confident enough about the results.