Add benchmarks

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I think it would make sense to add some benchmarks to detect performance regressions as we iterate on sim.

criterion is the tool usually recommended for this. Last I checked, it looked rather solid.

[ParensAllTheWayDown]

I would like to work on this. What would be an appropriate performance test?
Some thoughts. Given a fixed simple simulation.

• Measure the number of times a simulation can run in a fixed duration
• Measure the time to run events_int and events_ext for each model in simulation.