Rebase slice_group_by stabilization PR

Author: dtolnay

patches/0027-coretests-128bit-atomic-operations.patch

@@ -21,7 +21,7 @@ index 897a5e9..331f66f 100644
 -#![cfg_attr(target_has_atomic = "128", feature(integer_atomics))]
  #![cfg_attr(test, feature(cfg_match))]
  #![feature(int_roundings)]
- #![feature(slice_group_by)]
+ #![feature(split_array)]
 diff --git a/atomic.rs b/atomic.rs
 index b735957..ea728b6 100644
 --- a/atomic.rs