Add data structures used by redundant_clone

[Jarcho]

Contains just the data structures used in #14599

changelog: None

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[Jarcho]

@flip1995 Do you have any objections to running the data structure tests with miri? There's currently a few lines of unsafe code.

[flip1995]

Just saw this question. To also have the answer in the PR:

We discussed this in the last meeting. We should run the tests with miri, as long as it doesn't take too long, which it doesn't seem like it does. Also those tests should not run in the Rust repo (for now?), as it might be difficult to implement Clippy depending on Miri in rustc bootstrap.