Auto merge of #100645 - notriddle:notriddle/rustdoc-diet-plan, r=Guil…

…laumeGomez

rustdoc: strategic boxing to reduce the size of ItemKind and Type

The `Type` change redesigns `QPath` to box the entire data structure instead of boxing `self_type` and the `trait_`.

This reduces the size of several `ItemKind` variants, leaving `Impl` as the biggest variant. The `ItemKind` change boxes that variant's payload.