Freeze non-mutable readonly static dictionaries.

Since these will live for the lifetime of the process, it is worth freezing them on creation to improve lookup performance. If the dictionaries were iterated, change to an immutable array instead to preserve the order.

OpenRA.Game/Sync.cs

@@ -10,6 +10,7 @@
 #endregion
 
 using System;
+using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
@@ -40,7 +41,7 @@ namespace OpenRA
 			return GetHashFunction(sync)(sync);
 		}
 
-		static readonly Dictionary<Type, MethodInfo> CustomHashFunctions = new()
+		static readonly FrozenDictionary<Type, MethodInfo> CustomHashFunctions = new Dictionary<Type, MethodInfo>
 		{
 			{ typeof(int2), ((Func<int2, int>)HashInt2).Method },
 			{ typeof(CPos), ((Func<CPos, int>)HashCPos).Method },
@@ -53,7 +54,7 @@ namespace OpenRA
 			{ typeof(Actor), ((Func<Actor, int>)HashActor).Method },
 			{ typeof(Player), ((Func<Player, int>)HashPlayer).Method },
 			{ typeof(Target), ((Func<Target, int>)HashTarget).Method },
-		};
+		}.ToFrozenDictionary();
 
 		static void EmitSyncOpcodes(Type type, ILGenerator il)
 		{