Split opensolid/geometry into opensolid/primitives and opensolid/geometry?

[ianmackenzie]

opensolid/primitives would include points, vectors, directions, axes, planes, frames and bounding boxes, and opensolid/geometry would contain line segments, triangles, arcs, circles, splines, polylines and polygons. This would result in a much smaller/more focused opensolid/primitives library that might be more attractive to use in other Elm projects.