Rollup merge of #139656 - scottmcm:stabilize-slice-as-chunks, r=dtolnay

Stabilize `slice_as_chunks` library feature

~~Draft as this needs #139163 to land first.~~

FCP: rust-lang/rust#74985 (comment)

Methods being stabilized are:
```rust
impl [T] {
    const fn as_chunks<const N: usize>(&self) -> (&[[T; N]], &[T]);
    const fn as_rchunks<const N: usize>(&self) -> (&[T], &[[T; N]]);
    const unsafe fn as_chunks_unchecked<const N: usize>(&self) -> &[[T; N]];
    const fn as_chunks_mut<const N: usize>(&mut self) -> (&mut [[T; N]], &mut [T]);
    const fn as_rchunks_mut<const N: usize>(&mut self) -> (&mut [T], &mut [[T; N]]);
    const unsafe fn as_chunks_unchecked_mut<const N: usize>(&mut self) -> &mut [[T; N]];
}
```

~~(FCP's not done quite yet, but will in another day if I'm counting right.)~~ FCP Complete: rust-lang/rust#74985 (comment)

Author: ChrisDenton

tests/pass/slices.rs

@@ -1,7 +1,6 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
 //@compile-flags: -Zmiri-strict-provenance
-#![feature(slice_as_chunks)]
 #![feature(slice_partition_dedup)]
 #![feature(layout_for_ptr)]
 
@@ -227,7 +226,7 @@ fn test_for_invalidated_pointers() {
 
     buffer.reverse();
 
-    // Calls `fn as_chunks_unchecked_mut` internally (requires unstable `#![feature(slice_as_chunks)]`):
+    // Calls `fn as_chunks_unchecked_mut` internally:
     assert_eq!(2, buffer.as_chunks_mut::<32>().0.len());
     for chunk in buffer.as_chunks_mut::<32>().0 {
         for elem in chunk {