Check for deep equals before using vanilla registry element

Could potentially solve #962, although I can't reproduce that issue

This may be caused by two different changes to packetevents:
1. We did a little memory optimization so only elements different to the vanilly registry get stored
2. We fixed an issue where some elements would not equal the "same" vanilla element

This could cause every custom element to just never be registered, so we now do explicit deep-equals checks before saving the vanilla element instead

api/src/main/java/com/github/retrooper/packetevents/util/mappings/SynchronizedRegistriesHandler.java

@@ -34,6 +34,7 @@
 import com.github.retrooper.packetevents.protocol.item.trimpattern.TrimPattern;
 import com.github.retrooper.packetevents.protocol.item.trimpattern.TrimPatterns;
 import com.github.retrooper.packetevents.protocol.mapper.CopyableEntity;
+import com.github.retrooper.packetevents.protocol.mapper.DeepComparableEntity;
 import com.github.retrooper.packetevents.protocol.mapper.MappedEntity;
 import com.github.retrooper.packetevents.protocol.nbt.NBT;
 import com.github.retrooper.packetevents.protocol.nbt.NBTCompound;
@@ -142,7 +143,7 @@ public interface NbtEntryDecoder<T> {
     }
 
     @ApiStatus.Internal
-    public static final class RegistryEntry<T extends MappedEntity & CopyableEntity<T>> {
+    public static final class RegistryEntry<T extends MappedEntity & CopyableEntity<T> & DeepComparableEntity> {
 
         private final IRegistry<T> baseRegistry;
         private final NbtEntryDecoder<T> decoder;
@@ -186,7 +187,7 @@ private void handleElement(
             if (element.getData() != null) {
                 // data was provided, use registry element sent over network
                 T value = this.decoder.decode(element.getData(), version, data);
-                if (!value.equals(copiedBaseEntry)) {
+                if (!value.deepEquals(copiedBaseEntry)) {
                     // only define decoded value if it doesn't match the base
                     // registry value this ensures we don't save everything twice,
                     // if it has been already stored in memory