Implement Scratch-like sequencer behavior

[adroitwhiz]

Depends on #210.

This PR makes a few changes as progress towards #212:

• Instead of stepping each script once per frame, we do what Scratch does and keep going until a redraw is requested or we run out of frametime.
• Therefore, requesting redraws has also been implemented. Most functions which cause something visual to change on-screen request a redraw.
• The state that corresponds to a running script (namely the generator) has been moved out of Trigger and into its own class called Thread. Threads can:
  • Be "parked"--if it is waiting for a promise to resolve, it will park itself until the promise resolves.
  • Yield to another thread--that is, if the other thread is parked, the yielding thread will park itself as well, and ask the other thread to wake it up.
  • Be restarted in-place, without worrying about stale promise state.

Generator functions that are running as Leopard threads can await promises without having to be async themselves using the new Thread.await function.