Rollup merge of rust-lang#109616 - jsha:href-template, r=notriddle

rustdoc: use a template to generate Hrefs

Previously, href always returned a string, so each link incurred at least one allocation. We were also using `"../".repeat()`, and `.collect()` into strings to generate part of the paths. This abstracts away those repetitions into a couple of helper structs, ParentDirectories and PathComponents, and puts them together into an `Href` template, which should save some allocations. As a side benefit, I think it makes the logic of constructing links a little clearer.

This removes the only call sites for certain methods of UrlPartsBuilder, so it also removes those methods.

Also, remove the ItemType return value from `href` and friends; it was basically unused.

Part of rust-lang#108868

Author: aliemjay

src/librustdoc/clean/types.rs

@@ -460,7 +460,8 @@ impl Item {
             .iter()
             .filter_map(|ItemLink { link: s, link_text, page_id: id, ref fragment }| {
                 debug!(?id);
-                if let Ok((mut href, ..)) = href(*id, cx) {
+                if let Ok((href, ..)) = href(*id, cx) {
+                    let mut href = href.render_string();
                     debug!(?href);
                     if let Some(ref fragment) = *fragment {
                         fragment.render(&mut href, cx.tcx())