Make a shape's Segment table use handles from the Point table

Replace `SegmentDomain::handles` with a `Curvature` enum which describes the Bezier handle(s) or other curvature info like None for a polyline, Cubic, Quadratic, or even Arc or Conic or Euler Spiral or NURBS or other forms of curve representations.

This is the first step in making all data be attribute-driven in #1832.