Cell graphs

[dm3]

I loved the idea expressed by @alandipert on Slack about making the cell graph first-class. It would be great if we could explore the space and maybe narrow down the design in this issue.

While working on the JVM Javelin support I've implemented a very basic reified graph support which allows me to have separate concurrency-safe graphs of cells on this branch.

The other benefits I see:

• Custom error propagation strategy per graph. E.g. you can set the graph to behave like Excel and propagate #ERR on formula errors instead of the default throwing behaviour.
• Graph destruction. Useful when doing live-reload of a web-app, otherwise the formula cells keep accumulating on every reload.
• Cleanly separating independent components without having them inadvertently interfere with cells in other parts of the application.