Is your feature request related to a problem? Please describe.
Programming with current ProtocolLib for a legacy server version 1.8.8 with its corresponding legacy PacketWrapper requires a magic number here and there too much.

Describe the solution you'd like
I'd like to have methods toLegacyId and fromLegacyId added in EnumWrappers.ItemSlot.
The javadoc should state that legacy means before introduction of offhand aka for server version 1.8.8.