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.Common/Lint/CheckTraitPrerequisites.cs

@@ -21,8 +21,8 @@ namespace OpenRA.Mods.Common.Lint
 			foreach (var actorInfo in map.Rules.Actors.Where(a => !a.Key.StartsWith("^")))
 				try
 				{
-					var traits = actorInfo.Value.TraitsInConstructOrder().ToArray();
-					if (traits.Length == 0)
+					var hasTraits = actorInfo.Value.TraitsInConstructOrder().Any();
+					if (!hasTraits)
 						emitWarning("Actor {0} has no traits. Is this intended?".F(actorInfo.Key));
 				}
 				catch (Exception e)