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

[Keavon]

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.