Previously, you'd ask this function 'What lies on the other side of
edge #i of this Spectre tile?' and it would tell you the identity of
another Spectre. Now it will also tell you which _edge_ of that
Spectre adjoins the specified edge of the input one. This will be used
in the extra spectre-test mode I'm about to add.

spectre-test will want to use these for an additional generation mode.

This uses a tile shape very similar to the hat, but the tiling
_structure_ is totally changed so that there aren't any reflected
copies of the tile.

I'm not sure how much difference this makes to gameplay: the two
tilings are very similar for Loopy purposes. But the code was fun to
write, and I think the Spectre shape is noticeably prettier, so I'm
adding this to the collection anyway.

The test programs also generate a pile of SVG images used in the
companion article on my website.