After the refactoring of the animation library, we were missing some
dartdoc.

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.

Just infer it from the sign of the velocity. That's what odeon does.

They settle much faster after you fling them now.

This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.

This required some changes to AnimationPerformance to better understand
animating with forces.