Add debug event instrumentation to turtle methods.

This change adds code to report turtle motion events to a parent frame
debugger, if present.  Also, certain methods will now take time to run,
to make them easier to understand in a debugger (for example, dot will
take time to draw the dot; and scale will take time to animate the
scaling).  Animated effects are added for each of these methods.
A few APIs are cleaned up by this change: namely "label" no longer
takes a nontrivial callback.

This change also adds a table of built-in characters for the turtle,
so you can say "wear red pencil" or "wear blue pointer" or
"wear silver radius".  Some of these characters are used for animated
effects.