Apply Clippy's `--fix` to a single file.

I've been testing clippy --fix since #5363 got merged, and it's working great ❤️.

I've been thinking on integrating it into my editor similar to how rustfmt runs when I save a file.

Currently, clippy --fix fixes all issues in a project. Running clippy against a single file isn't useful (or supported), since it needs the context of a single crate to be able to report certain problems (e.g. unused variables, etc).

However, a combination of "check entire crate", combined with "apply fixes for this one file", would make it possible to integrate Clippy in an editor by applying fixes to the active file, while still checking the entire crate for contextual information it needs.

I'm curious if this is something that sounds interesting to pursuit. I'm not sure what the UX around this feature would be, but I guess it would be to provide --fix, and also some kind of repeatable --fix-file to fix any issues in one or more files?

Start by reviewing the existing clippy --fix behavior and the discussion around issue #5363. Define how whole-crate checking should interact with applying fixes to selected files, including the proposed repeatable --fix-file option and editor workflow. Done should include an agreed command-line interface and behavior for one or more files.