apacheGH-3522: Optimize IntList.size() from O(slabs) to O(1) with running counter

IntList.size() was iterating over all slabs to sum their lengths on every
call. Replace with a simple totalSize counter incremented on each add().
This eliminates O(slabs) overhead from dictionary encoding hot paths where
size() is called frequently.

Author: iemejia

parquet-column/src/main/java/org/apache/parquet/column/values/dictionary/IntList.java

@@ -99,6 +99,7 @@ private void incrementPosition() {
   // not be added
   private int[] currentSlab;
   private int currentSlabPos;
+  private int totalSize;
 
   private void allocateSlab() {
     currentSlab = new int[currentSlabSize];
@@ -129,6 +130,7 @@ public void add(int i) {
 
     currentSlab[currentSlabPos] = i;
     ++currentSlabPos;
+    ++totalSize;
   }
 
   /**
@@ -150,11 +152,6 @@ public IntIterator iterator() {
    * @return the current size of the list
    */
   public int size() {
-    int size = currentSlabPos;
-    for (int[] slab : slabs) {
-      size += slab.length;
-    }
-
-    return size;
+    return totalSize;
   }
 }