Rollup merge of rust-lang#130078 - its-the-shrimp:rustdoc-types-compress-ids, r=aDotInTheVoid

rustdoc-json: change item ID's repr from a string to an int

Following [this discussion on Zulip](https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/Optimizing.20the.20.60Id.60.20type.20in.20.60rustdoc-types.60), I've changed the repr of `rustdoc_json_types::Id` from a String to a u32, by adding a `clean::ItemId` interner to `JsonRenderer`

r? ``@aDotInTheVoid``

Author: tgross35