TechTree.GatherOwnedPrerequisites performance improvements.

- Consuming methods cared only about the count and not the actual actors, so only counts the actors rather that creating lists.
- ProvidesPrerequisites implementations return cached objects rather then allocating new enumerables on each call.

OpenRA.Mods.Common/Traits/Player/ProvidesPrerequisite.cs

@@ -11,6 +11,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using OpenRA.Traits;
 
 namespace OpenRA.Mods.Common.Traits
@@ -39,7 +40,7 @@ namespace OpenRA.Mods.Common.Traits
 
 	public class ProvidesPrerequisite : ConditionalTrait<ProvidesPrerequisiteInfo>, ITechTreePrerequisite, INotifyOwnerChanged, INotifyCreated
 	{
-		readonly string prerequisite;
+		readonly string[] prerequisites;
 
 		bool enabled;
 		TechTree techTree;
@@ -48,24 +49,15 @@ namespace OpenRA.Mods.Common.Traits
 		public ProvidesPrerequisite(ActorInitializer init, ProvidesPrerequisiteInfo info)
 			: base(info)
 		{
-			prerequisite = info.Prerequisite;
-
-			if (string.IsNullOrEmpty(prerequisite))
-				prerequisite = init.Self.Info.Name;
+			if (string.IsNullOrEmpty(info.Prerequisite))
+				prerequisites = new[] { init.Self.Info.Name };
+			else
+				prerequisites = new[] { info.Prerequisite };
 
 			faction = init.GetValue<FactionInit, string>(init.Self.Owner.Faction.InternalName);
 		}
 
-		public IEnumerable<string> ProvidesPrerequisites
-		{
-			get
-			{
-				if (!enabled)
-					yield break;
-
-				yield return prerequisite;
-			}
-		}
+		public IEnumerable<string> ProvidesPrerequisites => enabled ? prerequisites : Enumerable.Empty<string>();
 
 		protected override void Created(Actor self)
 		{