Do not create throw-away empty array for each trie level's Iterator

Author: ilya-g

kotlinx-collections-immutable/src/main/kotlin/kotlinx/collections/immutable/implementations/immutableMap/PersistentHashMapContentIterators.kt

@@ -3,7 +3,7 @@ package kotlinx.collections.immutable.implementations.immutableMap
 internal const val TRIE_MAX_HEIGHT = 7
 
 internal abstract class TrieNodeBaseIterator<out K, out V, out T> : Iterator<T> {
-    protected var buffer = emptyArray<Any?>()
+    protected var buffer = TrieNode.EMPTY.buffer
         private set
     private var dataSize = 0
     protected var index = 0

kotlinx-collections-immutable/src/main/kotlin/kotlinx/collections/immutable/implementations/immutableSet/PersistentHashSetIterator.kt

@@ -85,7 +85,7 @@ internal open class PersistentHashSetIterator<E>(node: TrieNode<E>) : Iterator<E
 }
 
 internal class TrieNodeIterator<out E> {
-    private var buffer = emptyArray<Any?>()
+    private var buffer = TrieNode.EMPTY.buffer
     private var index = 0
 
     fun reset(buffer: Array<Any?>, index: Int = 0) {