structural_simplify on an OptimizationProblem

[ChrisRackauckas]

https://github.com/SciML/GalacticOptim.jl/pull/237/files#r865796564

Constraints should probably have a way of being a NonlinearSystem for the constraint-satisfaction problem. In fact, structural_simplify should have the interpretation of tearing and simplification on that constraint-satisfaction system, i.e. if x ~ y is a constraint then it should just be eliminated from the system. What you could do is transform the OptimizationSystem to the NonlinearSystem of the constraints, apply tearing, and then grab the observed equations and reduced system, and make that be the reduced system of the OptimizationSystem.