Feature proposal: Saving symbolic derivatives

@odow I am starting discussion about new feature for MathOptSymbolicAD.jl .
I think adding option to save symbolic derivative expressions to file would be useful.

Many of us develop applications where we do not care about computation time of parts that can be computed only once beforehand. Lets enable saving that since it does not change depending on the model data. This should eliminate some drawbacks of symbolic approach for many use cases.