* Define session data structure and some operations
* Make code evaluation request async, so that we don't need an intermediary process
* Simplify id typespecs
* Make operation application composable
* Keep a separate evaluation queue per section and actually support concurrent evaluation
* Small fixes
* Validate queued cell type and set evaluation timestamp
* Apply review suggestions
* Add tests
* Store evaluating_cell_id instead of section status
* Add dynamic supervisor for managing evaluator processes
* Some fixes
* Refactor operation application
* Upon cell deletion mark dependent cells as stale
