Change Read/Show instances, update picklers and change LangTag exports

The reason for removing these instances are the same as in #24.

Affected types are: StanzaID, IQRequestType, MessageType,
PresenceType, StanzaErrorType, StanzaErrorCondition, SaslError,
StreamErrorCondition, Version, and LangTag.

It's quite boilerplate and ugly, but I think that it will do for now.