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.

[PullJosh]

I've sat on this and a few other PRs for a few days because I've had other things going on. What I'm realizing is that as fall approaches I need to dedicate more of my time and energy to preparing for my first year of full-time teaching (high school math), which means less time for Leopard.

Leopard is a hobby for me, and I'm sure from time to time throughout the school year I will continue to work on it for fun, but I'm going to take a break from the responsibility I assigned to myself over the summer, which was to proactively work and respond to issues, PRs, and user feedback.

What that means practically, for this PR, is that I will look at it eventually, but I'm not going to make any promises about exactly when. If two trusted parties want to review each other's work and contribute without my involvement (such as @towerofnix and @adroitwhiz) that is, of course, lovely and encouraged.

[towerofnix]

Thanks @PullJosh — we're in a bit of a similar boat too... it's sort of like this for everyone, sometimes, but there just ends up being stuff you're part of beyond the projects that still really matter to you, right? We're evaluating and working on our priorities right now!! And clearly you are too. It counts for a lot!

So, same here! We totally want to keep working on stuff here. (And in our other projects, too.) But it's as time allows, and as we're able to find a place among the rest of life, y'know. We love reviewing code and we also love testing it - it's just a question of where and when that fits, for us, which we'll keep figuring out.

Thanks for the heads up and for sharing where you're at, it means a lot!!

Edit edit: Also, good luck with your first year teaching, seriously!! And congrats embarking on that ✨

[PullJosh]

Thank you! I'm very excited! 😃

And I can totally imagine a world where Leopard continues to make progress just at an inconsistent rate as we occasionally have the time, energy, and desire to do something for a bit here and there.