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.

Start by locating SegmentDomain::handles and the Point table, then read the attribute-driven work tracked in #1832. Define the scope of the Curvature representation from the issue before changing the data model. Done means SegmentDomain::handles is replaced and the supported curvature forms, including None, Cubic, and Quadratic, are represented consistently.

索引モデルが issue の本文から書いたものです。