* Add multi-attribute aliases to some packets, for user convenience.
* Add support for writing PlayerListItemPacket.
* Add 'fields' attributes to manually-read/written packet classes,
implementing 'field_string' where appropriate to allow enable the
default __repr__ implementation.
* Modify data constructors where appropriate so that __repr__
implementations match their constructor protocols.
* Improve comments on type aliases within packet classes.
* Add/modify tests to cover the new functionality.