SKIP_SERIALIZE_READ_PANIC `skip_serialize` read implementations panic!

[gtker]

SKIP_SERIALIZE_READ_PANIC `skip_serialize` read implementations panic!

[gtker]

The SMSG_ADDON_INFO definition has been changed to use a custom type instead of the skip_serialize tag since this message is the only one that requires this work around for Vanilla/TBC/Wrath.

Using Predrag's guide to sealed traits, the Message trait should be a private trait to prevent downstream crates from using it at all.
The only reason it exists is to prevent code duplication in the different major version Server/ClientMessage implementations.
Since calling read_body can panic for the Wrath version of SMSG_ADDON_INFO it is not safe to use without explicitly ensuring that this won't be called.
This can be done internally in the expected helper functions but would probably not be doable outside of the crate.

The Server/ClientMessage traits should be supertrait sealed trait, since downstream should be able to use it but not impl it.