New redundant boxed types lint

[whirm]

fixes #2394

changelog: [redundant_box]: Lint on unnecessarily boxed types (sized types no bigger than a thin pointer).

[blyxyas]

Hi @whirm
In this repository we use a bot to automatically assign reviews at semi-random, so pull requests marked as "draft" usually fly under the radar as they aren't asigned to anyone. Could you fix CI and verify if this is still in draft period?

[whirm]

@blyxyas It's still not ready to be merged as there's a case I want to cover but I'm still investigating how to approach it.

I could use some help figuring out how to allow for this case:

fn smaller_than_usize_is_ok_to_box_if_using_into_raw() {
    let boxed = Box::new(1u8);
    let ptr = Box::into_raw(boxed);
}

Where boxed would usually be linted but not in this case, as it's converted into a wrapped raw pointer later on.

There's also the missing description that I'll do last in case there's changes on the lint itself before it's ready.

Shall I go ahead mark it as ready anyway?

Thanks!

[rustbot]

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