Implement PointerLike for NonNull and isize.

`PointerLike` is already implemented for raw pointers and `usize`,
so this should be straightforward.

Author: kpreid

library/core/src/marker.rs

@@ -997,11 +997,13 @@ pub trait PointerLike {}
 marker_impls! {
     #[unstable(feature = "pointer_like_trait", issue = "none")]
     PointerLike for
+        isize,
         usize,
         {T} &T,
         {T} &mut T,
         {T} *const T,
         {T} *mut T,
+        {T} crate::ptr::NonNull<T>,
         {T: PointerLike} crate::pin::Pin<T>,
 }