Rollup merge of #129449 - coolreader18:pin-as_deref_mut-signature, r=dtolnay

Put Pin::as_deref_mut in impl Pin<Ptr> / rearrange Pin methods

Tracking issue: #86918

Based on the suggestion in https://github.com/rust-lang/rust/issues/86918#issuecomment-2189367582

> Some advantages:
>
>  * Synergy with the existing `as_ref` and `as_mut` signatures (stable since Rust 1.33)
>
>  * Lifetime elision reduces noise in the signature
>
>  * Turbofish less verbose: `Pin::<&mut T>::as_deref_mut` vs `Pin::<&mut Pin<&mut T>>::as_deref_mut`

The comment seemed to imply that `Pin::as_ref` and `Pin::as_mut` already share an impl block, which they don't. So, I rearranged it so that they do, and we can see which looks better in the docs.

<details><summary><b>Docs screenshots</b></summary>

Current nightly:
![image](https://github.com/user-attachments/assets/b432cb82-8f4b-48ae-bafc-2fe49d0ad48c)

`Pin::as_deref_mut` moved into the same block as `as_mut`:
![image](https://github.com/user-attachments/assets/f9b35722-6a88-4465-ad1c-28d8e91902ac)

`Pin::as_ref`, `as_mut`, and `as_deref_mut` all in the same block:
![image](https://github.com/user-attachments/assets/9a1b2bf0-70a6-4751-b13f-390f1d575244)

</details>

I think I like the last one the most; obviously I'm biased since I'm the one who rearranged it, but it doesn't make sense to me to have `as_ref` methods split up by an `into_inner` method.

r? dtolnay

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