Use a BitSet for representing target types.

- Rename Bits<T> to BitSet<T>.
- Implement set based helpers for BitSet<T>.
- When representing TargetTypes of ITargetable in various traits, use a BitSet<TargetableType> instead of HashSet<string> for better performance & reduced memory usage.
- Fix FieldLoader to trim input values when generating a BitSet<T>.
- Require T in BitSet<T> and BitSetAllocator<T> to be a class since it's just a marker value. This allows the JIT to instantiate generic code for these classes once, as they don't benefit from specialized code for T. (Typically JITs will generate shared code for all reference types, and unique code for every value type encountered).

OpenRA.Mods.Common/Traits/AutoTargetPriority.cs

@@ -9,7 +9,7 @@
  */
 #endregion
 
-using System.Collections.Generic;
+using OpenRA.Primitives;
 using OpenRA.Traits;
 
 namespace OpenRA.Mods.Common.Traits
@@ -18,10 +18,10 @@ namespace OpenRA.Mods.Common.Traits
 	public class AutoTargetPriorityInfo : ConditionalTraitInfo, Requires<AutoTargetInfo>
 	{
 		[Desc("Target types that can be AutoTargeted.")]
-		public readonly HashSet<string> ValidTargets = new HashSet<string> { "Ground", "Water", "Air" };
+		public readonly BitSet<TargetableType> ValidTargets = new BitSet<TargetableType>("Ground", "Water", "Air");
 
 		[Desc("Target types that can't be AutoTargeted.", "Overrules ValidTargets.")]
-		public readonly HashSet<string> InvalidTargets = new HashSet<string>();
+		public readonly BitSet<TargetableType> InvalidTargets;
 
 		[Desc("ValidTargets with larger priorities will be AutoTargeted before lower priorities.")]
 		public readonly int Priority = 1;