We don't need to have the derivative as a curve,
it is enough for us to compute values of the
derivative at a given t, which we can also do
for conics.
Arcs were appealing, but they have a fatal flaw: we can't
split our arcs without changing the ellipse they trace.
That could be fixed by adding an extra parameter, but then
it is no longer any better than conics.
So switch back to conics, which have the advantage that they
are used elsewhere.
Add a new curve type for elliptical arcs
and use it for rounded rectangles and circles.
We use the 'E' command to represent elliptical
arcs in serialized paths.
