svg.Bezier3: switch back to six-number implementation (can optimize later).
add interface svg.creator.Bezier3 to prepare for Bezier API change.
simplify construction of svg.Bezier3 and svg.ContinuedBezier3.
add interfaces svg.creator.{Arc,Bezier2}.
simplify construction of svg.Bezier2 and svg.ContinuedBezier2.
simplify construction of svg.Arc.
svg.(Continued)?Bezier[2,3]: move path data into common base class.
svg.(Continued)?Bezier[2,3]: move methods to(), toPoint() to common base class.
demo: put a dot at end of line, not beginning.
svg.{Bezier2,ContinuedBezier3}: add bound-point methods.
svg.ContinuedBezier[2,3]: remove unused fields.