Support for agent-based modeling in CodeWorld

One thing that is attractive, but remains difficult to do in CodeWorld today, is agent-based modeling. Of course the `activityOf` API is sufficient, but it involves a lot of complicated list manipulation to manage a multi-agent simulation in that way.

I'm interested in what a purely functional API for agent-based modeling would look like. One possible resource is https://www.researchgate.net/publication/228900479_FABLES_A_Functional_Agent-Based_Language_for_Multi-Agent_Simulations - it talks about using functional programming for agent-based modeling, and cites as an advantage that the implementation of an agent model in FABLES looks very similar to the mathematical description of the models. That sounds quite promising.

This is very much an open-ended design project. I'm not sure where it will lead, if anywhere.