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/SpriteLoaders/ShpTDLoader.cs

@@ -203,14 +203,14 @@ namespace OpenRA.Mods.Common.SpriteLoaders
 
 		public static void Write(Stream s, Size size, IEnumerable<byte[]> frames)
 		{
-			var compressedFrames = frames.Select(f => Format80.Encode(f)).ToArray();
+			var compressedFrames = frames.Select(f => Format80.Encode(f)).ToList();
 
 			// note: end-of-file and all-zeroes headers
-			var dataOffset = 14 + (compressedFrames.Length + 2) * 8;
+			var dataOffset = 14 + (compressedFrames.Count + 2) * 8;
 
 			using (var bw = new BinaryWriter(s))
 			{
-				bw.Write((ushort)compressedFrames.Length);
+				bw.Write((ushort)compressedFrames.Count);
 				bw.Write((ushort)0);
 				bw.Write((ushort)0);
 				bw.Write((ushort)size.Width);