[Feature] Improved SymbolicUtils.jl integration

[MilesCranmer]

I think the interface with SymbolicUtils.jl could be expanded in some ways. Some ideas:

• Pass simplification rules to Options as a SymbolicUtils.RuleSet. This would allow simplification to be done even with custom operators.
• Have a time limit on simplification. If the time limit passes, the original expression is returned.
  • I found that some expressions took exponentially longer to simplify. Having a simple time limit would fix this.
• Randomly simplify a subexpression, rather than the entire expression, during the simplification mutation. This would also help with the long simplification time of more complex expressions.

What do you think @AlCap23 @ChrisRackauckas?