Race condition on smart cell evaluation about livebook

[jonatanklosko]

Synchronizing the binding scanning is tricky, because there are multiple levels of asynchronicity. When cell evaluation finishes, we asynchronously ask the evaluator to run the scan_binding callback, from where the kino sends a message to itself, then in handle_info it may change attributes and send new source to Livebook. To make it sync, we would need to wait for all that before finishing evaluation, which I think is too much (and would require many changes, including Kino APIs).

That said, I think smart cells should handle the initial/empty environment more gracefully and produce roughly the same code as when persisted. In this particular case, see livebook-dev/kino_explorer#159.