Implement Rust Backend

[s3alfisc]

• Implement a Rust Backend for PyFixest.
• Reorganize demeaner_backend code logic.

[schroedk]

@s3alfisc it was nice to meet you at the PyCon some weeks ago (we met at the appliedAI booth together with @janosg). I have some time to work on this topic and I would have some questions, before starting. First of all, do you want the user to choose the backend? Or is it a question of benchmarking, i.e. if rust implementation is faster everywhere, then get rid of the numba dependency completely?

[s3alfisc]

Hi @schroedk, that's super awesome and I'm really excited! The idea was to initially support rust as one of three backends - we currently support numba (which is the default) and JAX for runs on the GPU. There I would like to add rust as an option. Eventually (maybe even directly after merging this pr), I would like to make Rust the default backend, still support JAX, and deprecate numba after a while. And in the meantime, we can use the rust + numba backends for easy benchmarking of one implementation against the other.