Lint using identity into_iter conversion

[aaudiber]

This should address #2887.

I'd like to test further by running the lint against some existing codebases. Looking for guidance on how to run the Clippy I build locally against another local repo.

[matthiaskrgr]

Hi, cd into the project you want to check, and then
path/to/clippy/repo/target/debug/cargo-clippy clippy might do the trick :)

[aaudiber]

Tried it out in the rustfmt, rls, and rustfix repos, as well as rust-clippy itself, but didn't find any occurrences of using into_iter() on an iterator. If I introduce such an error, the warning does appear.