Rust Engine: add a Backend trait

[W95Psp]

Context

Before this PR, we had no proper notion of a backend, only a notion of what is a printer.
A backend is a printer + some transformation phases.

This PR

This PR introduces a Backend trait, and a Phase trait.
The Phase trait is mostly inert for now, it requires some more design. (especially regarding phase constraints)

This PR also changes slightly the Printer and PrettyAst traits, and adds a few helpers.

[clementblaudeau]

LGTM (with a few questions). It is a much needed distinction between a printer (Ast->Doc) and a backend (set of phases + printer), and should make it much easier to create new backends, possibly by borrowing existing printers

[franziskuskiefer]

Generally lgtm. Just a couple nits.

And please add clippy to the CI. There are way too many warnings here that (are not from this PR).

[W95Psp]

Thanks for the review @franziskuskiefer, regarding clippy, I opened #1613

[franziskuskiefer]

Let's address the public API question separately. Sounds like that needs some design first.

[W95Psp]

Putting this in merge queue again. GH runner got stuck, it seems like: https://github.com/cryspen/hax/actions/runs/17139918692/job/48624921344.