Useful packages for (sparse) differentiation?

Hey there friends of JSO!

@adrhill and I have been hard at work on two autodiff toolkits which you might find useful:

- [DifferentiationInterface.jl](https://github.com/gdalle/DifferentiationInterface.jl), an interface to every possible autodiff package with support for derivative, gradient, jacobian, hessian, JVP, VJP, HVP. It also supports sparse autodiff with a coloring-based approach.
- [SparseConnectivityTracer.jl](https://github.com/adrhill/SparseConnectivityTracer.jl), a much faster alternative to Symbolics.jl for sparsity pattern detection.

We would be happy to discuss potential uses, and integration with your ecosystem. We're having similar talks with Optimization.jl and the SciML organization at large.

Ping @amontoison @abelsiqueira since we've already chatted a few times :)