CircleSegment: Add inner_arc and outer_arc

[waywardmonkeys]

No description provided.

[waywardmonkeys]

Draft because:

• Depends on Add Arc::flipped #367, Arc::flipped for a doc comment link.
• Am I right to have the inner_arc flipped from the outer_arc?
• Should we add similar things for the two radial lines connecting the arcs?

[richard-uk1]

LGTM

• I think it's right that the direction matches the way the arc is drawn
• we can always add radial lines later (and personally I doubt they're useful)

[waywardmonkeys]

I thought it might be useful to have the line for the inner and outer chords ... but if we expose inner / outer start and end points (rather than just center + radius + angle), then we can more readily construct whatever.

[waywardmonkeys]

(but we could also do that from the arc ... let the arc expose start / end points or a chord)