Target ModelingToolkit?

Hey Micah,
Nice work. When I look at the code though:

https://github.com/mehalter/Petri.jl/blob/master/src/solvers.jl#L29-L43

I can see that the generated vector field is looping through all possible values at runtime. I think it might make sense to use some program staging here, since Petri can build some pretty big models pretty fast, and this is going to grow exponentially. If you use this process to build an MTK model though, and then have MTK output the ODE function (and some helpers like Jacobians and stuff), then the compiled versions would omit the zero'd part of these tensors, which could have a pretty wild reduction in the amount of computation that has to be done. Let me know if you want any help doing this.