Import formatting in code

There is currently no import formatting rules in the codebase; adding import formatting could improve code maintainability, reducing the potential conflicts between contributions, as well improving code esthetics (for what it's worth, I know that this point is quite subjective).

rustfmt support import formatting, but the configurations options are still unstable. There are mainly two options: import granularity and group imports.
Also, contrary to other Rust component, it's possible to use unstable rustfmt options, but only using CLI, so formatting imports can be on stable Rust using for example --config "unstable_features=true,imports_granularity=Crate,group_imports=StdExternalCrate".

The downside of enabling import formatting is that it requires for contributors to setup their development environment with these options, or face a red CI. Adding a pre-commit hook can greatly help for that, and the fact that stable Rust can be used, so it doesn't requires contributor to download nightly Rust, makes the pre-commit hook worth considering.

Start by reviewing the repository's current Rust formatting and CI setup, then evaluate the rustfmt imports_granularity and group_imports options described in the issue. Decide how formatting would be applied for contributors, including whether a pre-commit hook is appropriate; done means the project has an agreed, reproducible import-formatting workflow.