Auto merge of #531 - ToMe25:optimize_is_disjoint, r=Amanieu

Optimize Set is_disjoint

By using the `Intersection` iterator in `HashSet::is_Disjoint` its performance can be significantly improved in some cases.
This is because `intersection()` always uses the shorter set as its iterator.

It would also be possible to replicate this "Iterate over the smaller set and check in the larger set" logic in the is_disjoint method.
However in my benchmarks the approach I chose is faster than that.

This change only causes a significant improvement when called on the larger of two disjoint sets.

My benchmark results:

Name | Before | After | Diff (%)
-- | -- | -- | --
disjoint_is_disjoint_large_small | 1,147.43 | 535.25 | -53,35 %
disjoint_is_disjoint_small_large | 535.66 | 527.59 | -1,51 %
subset_is_disjoint | 9.90 | 10.44 | 5,45 %
superset_is_disjoint | 9.80 | 10.43 | 6,43 %

src/set.rs

@@ -1044,7 +1044,7 @@ where
     /// assert_eq!(a.is_disjoint(&b), false);
     /// ```
     pub fn is_disjoint(&self, other: &Self) -> bool {
-        self.iter().all(|v| !other.contains(v))
+        self.intersection(other).next().is_none()
     }
 
     /// Returns `true` if the set is a subset of another,