Adding some tests for ReferenceStructures. They still don't work.
Flipped map semantics around to right-mapping when ranges are involved, and adjusted everything to compensate. There's some strangeness in merging still, where I have to invert the face of the Position I'm merging into in order to get the right answer back. Probably it's because ranges now need to represent right contexts on the forward strand instead of left ones. TODO: clear up wierdness with context side and strand by maybe flipping everything to downstream contexts everywhere