Auto merge of #7330 - xFrednet:0000-refactor-map-identity, r=flip1995

Auto merge of #7330 - xFrednet:0000-refactor-map-identity, r=flip1995

Refactoring identity function lints

I've noticed that we have several lints that all check for identity functions and each used their own check implementation. I moved the `is_expr_identity_function` function to `clippy_utils` and adapted all lints to reuse that one function. This should make the addition of new lints like this also easier in the future.

I've also moved the `map_identity` lint into the `methods` module. It's probably the best to review this PR by checking each commit individually. And that's it, have a great day :upside_down_face:

changelog: none