TODO

• Fix GC assertion failure!
• Cache output of derivers.
• Rederiving in background threads kills interactive latency. Try to make it do less work, be smarter about kicking it off, and generate less garbage. Unfortunately no thread priorities.
• Make signals lazy.
• Efficiently store lots of control events.
• Unify Signal with [event]?
• Pipe output of derivers into a block for editing... leads to merge problem. Can use event_stack as an ID?

• testing - I don't understand how to use quickcheck
  Example: Signal.track_signal
• There is a lot of [event] passed around, with the assumption that they're sorted. It would be nice to put that in the type. Similar to Map.fromList and Map.fromAscList, I often have a non-lazy unsorted version and a lazy sorted or sorted-preserving versions of a map function.
• GADTs for TrackLang? Typechecking and function calling is slightly unsatisfactory.