Move our current Rust-ish printer to the generic printer

[W95Psp]

https://github.com/hacspec/hax/blob/4291b195f4dee2bec5568ee6a0b6fe6a108623fb/engine/lib/print_rust.ml defines a Rust printer for our AST, we should move this implementation to the generic printer.

Currently, this printer does no pretty printing at all itself, but leverages rustfmt to do so. However, this printer outputs not only a string, but a list strings coupled with annotations to link to our AST, so that we can debug the phases in the web debugger.

This rustfmt trick uses re, which is causing problems (see #1106).

The generic printer uses pprint, which allows for easy pretty printing. So we should be able to have a decent output, sufficiently pretty for debugging.