Add missing stabilized slice chunk methods

Author: jieyouxu

RELEASES.md

@@ -64,6 +64,12 @@ Stabilized APIs
 - [`proc_macro::Span::end`](https://doc.rust-lang.org/stable/proc_macro/struct.Span.html#method.end)
 - [`proc_macro::Span::file`](https://doc.rust-lang.org/stable/proc_macro/struct.Span.html#method.file)
 - [`proc_macro::Span::local_file`](https://doc.rust-lang.org/stable/proc_macro/struct.Span.html#method.local_file)
+- [`[T]::as_chunks`](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.as_chunks)
+- [`[T]::as_rchunks`](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.as_rchunks)
+- [`[T]::as_chunks_unchecked`](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.as_chunks_unchecked)
+- [`[T]::as_chunks_mut`](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.as_chunks_mut)
+- [`[T]::as_rchunks_mut`](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.as_rchunks_mut)
+- [`[T]::as_chunks_unchecked_mut`](https://doc.rust-lang.org/stable/std/primitive.slice.html#method.as_chunks_unchecked_mut)
 
 These previously stable APIs are now stable in const contexts: