Switch impls of is_sorted_by between slices and slice iters

This makes a bit more sense — iter impl converts to slice first, while
slice impl used to create iter, doing unnecessary conversions.

library/core/src/slice/iter.rs

@@ -132,7 +132,7 @@ fn is_sorted_by<F>(self, mut compare: F) -> bool
         Self: Sized,
         F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
     {
-        self.as_slice().array_windows().all(|[a, b]| compare(&a, &b).map_or(false, Ordering::is_le))
+        self.as_slice().is_sorted_by(|a, b| compare(&a, &b))
     }
 }}
 

library/core/src/slice/mod.rs

@@ -3822,7 +3822,7 @@ pub fn is_sorted_by<'a, F>(&'a self, mut compare: F) -> bool
     where
         F: FnMut(&'a T, &'a T) -> Option<Ordering>,
     {
-        self.iter().is_sorted_by(|a, b| compare(*a, *b))
+        self.array_windows().all(|[a, b]| compare(a, b).map_or(false, Ordering::is_le))
     }
 
     /// Checks if the elements of this slice are sorted using the given key extraction function.