Cache ICrushable traits in actor.

OpenRA.Game/Traits/TraitsInterfaces.cs

@@ -649,4 +649,14 @@ namespace OpenRA.Traits
 	{
 		void PlayerDisconnected(Actor self, Player p);
 	}
+
+	// Type tag for crush class bits
+	public class CrushClass { }
+
+	[RequireExplicitImplementation]
+	public interface ICrushable
+	{
+		bool CrushableBy(Actor self, Actor crusher, BitSet<CrushClass> crushClasses);
+		LongBitSet<PlayerBitMask> CrushableBy(Actor self, BitSet<CrushClass> crushClasses);
+	}
 }