Improve worst-case performance of HashSet.is_subset

Ported from rust-lang/rust#59665

src/external_trait_impls/rayon/set.rs

@@ -256,7 +256,11 @@ where
     ///
     /// This method runs in a potentially parallel fashion.
     pub fn par_is_subset(&self, other: &Self) -> bool {
-        self.into_par_iter().all(|x| other.contains(x))
+        if self.len() <= other.len() {
+            self.into_par_iter().all(|x| other.contains(x))
+        } else {
+            false
+        }
     }
 
     /// Returns `true` if the set is a superset of another,

src/set.rs

@@ -640,7 +640,11 @@ where
     /// assert_eq!(set.is_subset(&sup), false);
     /// ```
     pub fn is_subset(&self, other: &Self) -> bool {
-        self.iter().all(|v| other.contains(v))
+        if self.len() <= other.len() {
+            self.iter().all(|v| other.contains(v))
+        } else {
+            false
+        }
     }
 
     /// Returns `true` if the set is a superset of another,