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.

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