Fix the comparison of union set sizes

Author: cuviper

src/set.rs

@@ -736,7 +736,9 @@ where
     /// ```
     #[cfg_attr(feature = "inline-more", inline)]
     pub fn union<'a>(&'a self, other: &'a Self) -> Union<'a, T, S, A> {
-        let (smaller, larger) = if self.len() >= other.len() {
+        // We'll iterate one set in full, and only the remaining difference from the other.
+        // Use the smaller set for the difference in order to reduce hash lookups.
+        let (smaller, larger) = if self.len() <= other.len() {
             (self, other)
         } else {
             (other, self)