Parity between animated shapes and Kurbo

It would be nice to have animated versions of more of the Kurbo shapes like `Circle`, `CircleSegment`, etc.

Also, existing shapes in Interpoli could be more like Kurbo:

* The structure of `Ellipse` is different
* `Rect` is actually a rounded rect.

The issue names no files or tests and describes a broad parity effort. Start by inventorying Interpoli's existing animated shapes and comparing them with Kurbo's Circle, CircleSegment, Ellipse, and Rect structures. Done requires an agreed shape scope plus matching animated shapes and the requested Ellipse and Rect structure changes.

Written by the indexing model from the issue text.