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/Traits/Util.cs

@@ -130,10 +130,7 @@ namespace OpenRA.Traits
 
 		public static IEnumerable<CPos> ExpandFootprint(IEnumerable<CPos> cells, bool allowDiagonal)
 		{
-			var result = new Dictionary<CPos, bool>();
-			foreach (var c in cells.SelectMany(c => Neighbours(c, allowDiagonal)))
-				result[c] = true;
-			return result.Keys;
+			return cells.SelectMany(c => Neighbours(c, allowDiagonal)).Distinct();
 		}
 
 		public static IEnumerable<CPos> AdjacentCells(World w, Target target)