rustdoc: simplify highlight.rs Split render_with_highlighting, which took many optional parameters, into three functions for specific purposes, which each take a smaller number of mostly required parameters. Remove some plumbing to pass through an "edition" parameter, which was used solely to avoid highlighting some 2021 Edition keywords in non-2021 code. I've tested a build of std docs before and after, and this does not change the generated HTML at all. Followup from https://github.com/rust-lang/rust/pull/91264#discussion_r901151101 r? ``@GuillaumeGomez``