Use SkipList iterator in ItemList instead of fastutil iterator (#556)

The fastutil iterator does not support external removal or pushing to the Set which might happen and will cause crashes

src/main/java/appeng/util/item/ItemList.java

@@ -179,7 +179,7 @@ public int size() {
     public Iterator<IAEItemStack> iterator() {
         return new MeaningfulItemIterator<>(new Iterator<>() {
 
-            private final Iterator<IAEItemStack> i = ItemList.this.setRecords.iterator();
+            private final Iterator<IAEItemStack> i = ItemList.this.records.values().iterator();
             private IAEItemStack next = null;
 
             @Override
@@ -195,7 +195,7 @@ public IAEItemStack next() {
             @Override
             public void remove() {
                 i.remove();
-                ItemList.this.records.remove(next);
+                ItemList.this.setRecords.remove(next);
             }
         });
     }