Consider migrating to diffrax

[Diffrax](https://github.com/patrick-kidger/diffrax) is a JAX-powered ODE solver from which the project might benefit.

**Please complete the following tasks**

- [x] I have searched the documentation and help resources before reporting.
- [x] I have searched the [discussions](https://github.com/geem-lab/overreact/discussions) to avoid duplicates.
- [x] I have searched the [open](https://github.com/geem-lab/overreact/issues) and [closed](https://github.com/geem-lab/overreact/issues?q=is%3Aissue+is%3Aclosed) issues to avoid duplicates.

**Is your feature request related to a problem? Please describe.**

The solver we currently use (SciPy) is not reliable enough, largely because our use cases are too stiff.

**Describe the solution you'd like**

Anything that Just Works® would be great.

**Describe alternatives you've considered**

Rewrite in Rust, perform some pre-equilibrations, tweak with SciPy options, etc. None of those should be as simple as the proposed one?

**Additional context**

The stiffest problems seem to be the ones with equilibria. Maybe we should test more stuff like that as well?