Graphical scheduling of rewrite rules

[kris-brown]

This PR implements a traced monoidal category which formalizes schedules of rewrite rules as wiring diagrams. The semantics is that, at any point in time, the world state lives on a wire in the diagram. Boxes have a general form, but for scheduling we consider a handful of primitive boxes, such as the application of a rewrite rule, a control flow function (e.g. flip a coin to go down one path vs another), a query box (for each match in the query, run a schedule focused on the result).

Not only does this let us construct large schedules compositionally and implement the scheduler in a modular manner, but we also visualize where we are in the schedule in lockstep with the trajectory of world states that results from a simulation, which adds helpful context.

This PR also adapts to the incorporations of variables into ACSet attributes.