Add PathBuilder Arc command
The change is a bit more involved than I thought it'd be. I had to add a few intermediary types to reuse the existing code. I cleaned up and commented the segmentation code a bit for readability.
One issue is the need for the start point of the arc. I tried using
EllipticalArc::to_cairo but the test suite failed when I did, so I stored it in
EllipticalArc as a workaround.
Closes #190 (closed).