simplify runtime set access

Assets/Scripts/Enemy.cs

@@ -131,7 +131,7 @@ private void OnDisable()
 
         private void Retarget()
         {
-            if (heroSet == null || heroSet.Count == 0)
+            if (heroSet.IsEmpty)
             {
                 // No heroes to target.
                 // TODO(zeph): switch to a special behavior?
@@ -140,13 +140,7 @@ private void Retarget()
             }
             // TODO(zeph): target based on threat, once threat exists
             var k = Random.Range(0, heroSet.Count);
-            var it = heroSet.GetEnumerator();
+            target = heroSet[k].gameObject;
-            // This seems to be the best way to do this...?
-            for (var i = 0; i < k; i++)
-            {
-                it.MoveNext();
-            }
-            target = it.Current.gameObject;
         }
 
         private void SetBehavior(BehaviorState behavior)

Assets/Scripts/RuntimeSet/RuntimeSetSO.cs

@@ -37,5 +37,7 @@ IEnumerator IEnumerable.GetEnumerator()
         {
             return GetEnumerator();
         }
+
+        public T this[int index] { get { return items[index]; } }
     }
 }