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.Game/Player.cs

@@ -50,7 +50,7 @@ namespace OpenRA
 		{
 			var selectableCountries = world.Map.Rules.Actors["world"].Traits
 				.WithInterface<CountryInfo>().Where(c => c.Selectable)
-				.ToArray();
+				.ToList();
 
 			return selectableCountries.FirstOrDefault(c => c.Race == name)
 				?? selectableCountries.Random(world.SharedRandom);