style: refactor import layout

[xtqqczze]

Configure rustfmt to use vertical import formatting and apply the new layout throughout the crate.

• Update rustfmt configuration and reformat imports throughout.
• Group related imports into multi-line use statements.

Use the following import order:

1. core::
2. std::
3. external crates
4. crate::
5. super::
6. self::

[xtqqczze]

@clarfonthey Doesn't look like cargo fmt --all -- --check is using nightly in ci/tools.sh

[clarfonthey]

Ah, I thought it was. Either way, that should be an easy change.

[xtqqczze]

@clarfonthey Right, I'm not completely sure how ci/tools.sh works, would you mind taking a look? And what do you think about the changes in principle?