Fix Box::leak should be public

Author: zakarumych

src/stable/boxed.rs

@@ -1216,7 +1216,7 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
     /// assert_eq!(*static_ref, [4, 2, 3]);
     /// ```
     #[inline(always)]
-    fn leak<'a>(b: Self) -> &'a mut T
+    pub fn leak<'a>(b: Self) -> &'a mut T
     where
         A: 'a,
     {