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/lib.rs

@@ -120,6 +120,7 @@
 #![feature(const_vec_new)]
 #![feature(slice_partition_dedup)]
 #![feature(maybe_uninit)]
+#![feature(rchunks)]
 
 // Allow testing this library