Add more fuzz tests for point/vector/direction functions

The Point#d, Vector#d and Direction#d modules were recently optimized in the coordinate-systems branch to avoid unnecessary object allocations and function calls, but this also meant most functions got more verbose and therefore more prone to typos. It would be very useful to add some more fuzz and/or hardcoded tests for the various functions in these modules, following the patterns in the existing Tests.Point3d etc. test modules. Please make sure you make any PRs against the coordinate-systems branch!