added concrete IEqualityComparer<ExtensionIntPair> implementation in ExtensionRegistryLite.cs to prevent AOT compilation issue with unity in iOS

csharp/src/ProtocolBuffers/ExtensionRegistryLite.cs

@@ -96,7 +96,7 @@ namespace Google.ProtocolBuffers
     {
         private static readonly ExtensionRegistry empty = new ExtensionRegistry(
             new ExtensionByNameMap(),
-            new ExtensionByIdMap(),
+            new ExtensionByIdMap(new ExtensionIntPairEqualityComparer()),
             true);
 
         private readonly ExtensionByNameMap extensionsByName;
@@ -116,7 +116,7 @@ namespace Google.ProtocolBuffers
         /// </summary>
         public static ExtensionRegistry CreateInstance()
         {
-            return new ExtensionRegistry(new ExtensionByNameMap(), new ExtensionByIdMap(), false);
+            return new ExtensionRegistry(new ExtensionByNameMap(), new ExtensionByIdMap(new ExtensionIntPairEqualityComparer()), false);
         }
 
         public ExtensionRegistry AsReadOnly()
@@ -216,5 +216,17 @@ namespace Google.ProtocolBuffers
                 return msgType.Equals(other.msgType) && number == other.number;
             }
         }
+
+        internal class ExtensionIntPairEqualityComparer : IEqualityComparer<ExtensionIntPair>
+        {
+            public bool Equals(ExtensionIntPair x, ExtensionIntPair y)
+            {
+                return x.Equals(y);
+            }
+            public int GetHashCode(ExtensionIntPair obj)
+            {
+                return obj.GetHashCode();
+            }
+        }
     }
 }