RecyclableList.IndexOf performance improvements.

Recyclable.Collections/RecyclableDefaults.cs

@@ -4,7 +4,10 @@
 	{
 		public const int BlockSize = 16_384;
 		public const int Capacity = 1;
+		public const long MinItemsCountForParallelization = 786_432;
 		public const int MinPooledArrayLength = 128;
 		public const int MaxPooledBlockSize = 2_147_483_591;
+		public const long ItemNotFoundIndexLong = -1L;
+		public const int ItemNotFoundIndex = -1;
 	}
 }