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.Game/GameRules/WeaponInfo.cs

@@ -13,6 +13,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using OpenRA.Effects;
+using OpenRA.Primitives;
 using OpenRA.Traits;
 
 namespace OpenRA.GameRules
@@ -64,10 +65,10 @@ namespace OpenRA.GameRules
 		public readonly int Burst = 1;
 
 		[Desc("What types of targets are affected.")]
-		public readonly HashSet<string> ValidTargets = new HashSet<string> { "Ground", "Water" };
+		public readonly BitSet<TargetableType> ValidTargets = new BitSet<TargetableType>("Ground", "Water");
 
 		[Desc("What types of targets are unaffected.", "Overrules ValidTargets.")]
-		public readonly HashSet<string> InvalidTargets = new HashSet<string>();
+		public readonly BitSet<TargetableType> InvalidTargets;
 
 		[Desc("Delay in ticks between firing shots from the same ammo magazine. If one entry, it will be used for all bursts.",
 			"If multiple entries, their number needs to match Burst - 1.")]
@@ -116,7 +117,7 @@ namespace OpenRA.GameRules
 			return retList;
 		}
 
-		public bool IsValidTarget(IEnumerable<string> targetTypes)
+		public bool IsValidTarget(BitSet<TargetableType> targetTypes)
 		{
 			return ValidTargets.Overlaps(targetTypes) && !InvalidTargets.Overlaps(targetTypes);
 		}