Use a LinkedHashSet to store keys in a strict KeySet

We need a data structure that both acts like a Set (queries membership efficiently), and has a stable iteration order.
LinkedHashSet seems to fit this criteria.

Author: natsukagami

library/src/scala/collection/Map.scala

@@ -216,7 +216,7 @@ transparent trait MapOps[K, +V, +CC[_, _] <: IterableOps[?, AnyConstr, ?], +C]
     */
   protected trait GenKeySet { this: Set[K] =>
     // CC note: this is unavoidable to make the KeySet pure.
-    private[MapOps] val allKeys = MapOps.this.keysIterator.toList
+    private[MapOps] val allKeys = MapOps.this.keysIterator.to(mutable.LinkedHashSet)
     // We restore the lazy behavior in LazyKeySet
     def iterator: Iterator[K] =
       allKeys.iterator