Time-travel debugging

[nwinter]

See the CodeCombat sister issue for context. Basically, Aether should be able to grab and serialize all the variable states we might want to see from a debugging perspective.

[nwinter]

I'm going to build this soon. If anyone wants to collaborate with me on it, let me know and we'll get into it.

[nwinter]

It's going well, although performance and edge cases need work.

[nwinter]

This is pretty much working in CodeCombat now that we're running the flow only lazily in a separate simulation and not being so dumb about serializing it.