Add slice::rchunks(), rchunks_mut(), rchunks_exact() and rchunks_exact_mut()

These work exactly like the normal chunks iterators but start creating
chunks from the end of the slice.

See #55177 for the tracking issue

src/liballoc/slice.rs

@@ -125,6 +125,8 @@ pub use core::slice::{from_ref, from_mut};
 pub use core::slice::SliceIndex;
 #[unstable(feature = "chunks_exact", issue = "47115")]
 pub use core::slice::{ChunksExact, ChunksExactMut};
+#[unstable(feature = "rchunks", issue = "55177")]
+pub use core::slice::{RChunks, RChunksMut, RChunksExact, RChunksExactMut};
 
 ////////////////////////////////////////////////////////////////////////////////
 // Basic slice extension methods