TLV support in derive macro may lead to invalid serialization

[dr-orlovsky]

TLV stream has to be the last data item in any message. This wouldn't be the case if a message has fields which are in turn structures with TLV fields, as was discovered by @crisdut in LNP-WG/lnp-node#66

Derive macro has somehow to prevent that from happening (not clear how though...).