Add support for coroutines

[ryanmoelter]

Some ideas:

• Have a suspending AnimatorSetBuilder.start() that suspends until the animation is complete
  • Handle cancellation, so animations stop when e.g. the current screen is hidden
• Maybe have a non-suspending AnimatorSetBuilder.startInScope(CoroutineScope) for when we don't want to call launch directly?
• Can refactor all of blend to be based on coroutines, and the regular, non-suspending start() can use GlobalScope under the hood
  • Should we deprecate non-suspending start()?