Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.
Similar to widgets.dart, rendering.dart exports the entire rendering layer.
Also, update the examples to use rendering.dart and widgets.dart. Also clean up
some exports so that the examples have more sensible imports.
Sadly, box.dart has grown much longer than 1000 lines. This patch splits it up
into several files based on the class hierarchy. Fortunately, many of these
classes are loosely coupled to each other.
