[stdlib] Set.intersection iterate over smaller set (#36678)

* [stdlib] Set.intersection iterate over smaller set

When intersecting two sets, it is beneficial to iterate over the smaller sized set of the two, and check membership on the other. This speeds up runtime dramatically for cases where the current set is significantly larger than the set you are intersecting against.

* Review comments - variable names, implicit swap

Author: BenziAhamed

stdlib/public/core/Set.swift

@@ -1223,8 +1223,10 @@ extension Set {
   @inlinable
   public __consuming func intersection(_ other: Set<Element>) -> Set<Element> {
     var newSet = Set<Element>()
-    for member in self {
-      if other.contains(member) {
+    let (smaller, larger) =
+      count < other.count ? (self, other) : (other, self)
+    for member in smaller {
+      if larger.contains(member) {
         newSet.insert(member)
       }
     }