Checked LINQ queries and collections for inefficiencies.

- Made Array.IndexOf available via extension method.
- Made ToHashSet extension method.
- Change collections queried often via Contains into sets.
- Avoid Count() extension if Count or Length property exist.
- Made Count() > 0 checks and variations calls to Any() instead.
- Don't call ToList/ToArray if there is no benefit to materializing the sequence.
- If the sequence does benefit from materialization, follow this general pattern:
  - Collection queried often via Contains use ToHashSet to speed up lookups.
  - Short lived variables use ToList. This is because ToArray requires an extra copy to output the final size.
  - Collections persisted into fields or for a long time use ToArray to minimize memory overhead.

OpenRA.Mods.RA/Traits/SupportPowers/ChronoshiftPower.cs

@@ -57,11 +57,11 @@ namespace OpenRA.Mods.RA.Traits
 		{
 			var range = ((ChronoshiftPowerInfo)Info).Range;
 			var tiles = Self.World.Map.FindTilesInCircle(xy, range);
-			var units = new List<Actor>();
+			var units = new HashSet<Actor>();
 			foreach (var t in tiles)
-				units.AddRange(Self.World.ActorMap.GetUnitsAt(t));
+				units.UnionWith(Self.World.ActorMap.GetUnitsAt(t));
 
-			return units.Distinct().Where(a => a.HasTrait<Chronoshiftable>() &&
+			return units.Where(a => a.HasTrait<Chronoshiftable>() &&
 				!a.TraitsImplementing<IPreventsTeleport>().Any(condition => condition.PreventsTeleport(a)));
 		}