Implement `slice::split_once` and `slice::rsplit_once` Feature gate is `slice_split_once` and tracking issue is #112811. These are equivalents to the existing `str::split_once` and `str::rsplit_once` methods.
Implement `slice::split_once` and `slice::rsplit_once` Feature gate is `slice_split_once` and tracking issue is #112811. These are equivalents to the existing `str::split_once` and `str::rsplit_once` methods.