rustdoc: Rearrange Item/ItemInner.

The `Item` struct is 48 bytes and contains a `Box<ItemInner>`;
`ItemInner` is 104 bytes. This is an odd arrangement. Normally you'd
have one of the following.

- A single large struct, which avoids the allocation for the `Box`, but
  can result in lots of wasted space in unused parts of a container like
  `Vec<Item>`, `HashSet<Item>`, etc.

- Or, something like `struct Item(Box<ItemInner>)`, which requires the
  `Box` allocation but gives a very small Item size, which is good for
  containers like `Vec<Item>`.

`Item`/`ItemInner` currently gets the worst of both worlds: it always
requires a `Box`, but `Item` is also pretty big and so wastes space in
containers. It would make sense to push it in one direction or the
other. #138916 showed that the first option is a regression for rustdoc,
so this commit does the second option, which improves speed and reduces
memory usage.

src/librustdoc/json/conversions.rs

@@ -43,7 +43,7 @@ impl JsonRenderer<'_> {
         let attrs = item.attributes(self.tcx, self.cache(), true);
         let span = item.span(self.tcx);
         let visibility = item.visibility(self.tcx);
-        let clean::Item { name, item_id, .. } = item;
+        let clean::ItemInner { name, item_id, .. } = *item.inner;
         let id = self.id_from_item(&item);
         let inner = match item.kind {
             clean::KeywordItem => return None,