Cleanup and optimize `render_impls` - take ownership of the `Vec<&Impl>` instead of copying into another alloc - reuse `ImplString` to do natural sort ordering - lazy formatting Somewhat of a follow-up to rust-lang/rust#157233 and rust-lang/rust#157179 (cc @nnethercote - thanks!) This kinda undoes https://github.com/rust-lang/rust/pull/157179/changes/f7c8bc2b13e6c4af20da615839feb6000f8cba6c but IMHO it makes more sense to be explicit about negative impl ordering, and also seems kinda wasteful to "render" the negativity into a string and rely on however ASCII decided to order characters. I can also undo this part, I think this PR is still a positive change even without it. r? @GuillaumeGomez LLM disclosure: I used LLM for reviewing my changes and making sure some assumptions I was making were correct (i.e. that `render_impls` will not render anything IFF the list of traits passed to it is empty)