new lint: getting last element of path via iter().last()

In my code I found something awful like this:

        let name = path
            .to_str()
            .unwrap()
            .split('/')
            .last()
            .unwrap()
            .to_string();

to get the last element of a path.

This could be rewritten without split

    let last = path.into_iter().last().unwrap().to_str().unwrap().to_owned();

but most idiomatic is probably

let last = path.file_name().unwrap().to_str().unwrap().to_string();

Start with the Rust examples in the issue and locate Clippy’s existing lint implementations and UI-test entry points. Determine the intended matching and diagnostic behavior for the split or iter().last() path pattern, then add coverage for the proposed path-based alternative; done when the relevant Clippy tests pass.