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:
`Pin::as_deref_mut` moved into the same block as `as_mut`:
`Pin::as_ref`, `as_mut`, and `as_deref_mut` all in the same block:
</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
e96faab70b