Rollup merge of #103800 - danielhenrymantilla:stabilize-pin-macro, r=dtolnay

Stabilize `::{core,std}::pin::pin!`

As discussed [over here](rust-lang/rust#93178 (comment)), it looks like a decent time to stabilize the `pin!` macro.

### Public API

```rust
// in module `core::pin`

/// API: `fn pin<T>($value: T) -> Pin<&'local mut T>`
pub macro pin($value:expr $(,)?) {
    …
}
```

  - Tracking issue: #93178

(now all this needs is an FCP by the proper team?)

Author: compiler-errors

tests/pass/issues/issue-miri-2068.rs

@@ -1,5 +1,3 @@
-#![feature(pin_macro)]
-
 use core::future::Future;
 use core::pin::Pin;
 use core::task::{Context, Poll};

tests/pass/stacked-borrows/future-self-referential.rs

@@ -1,5 +1,3 @@
-#![feature(pin_macro)]
-
 use std::future::*;
 use std::marker::PhantomPinned;
 use std::pin::*;