* 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.
This commit introduces two backward-incompatible changes which may break
existing code:
(1) `networking.packets.clientbound.play.SpawnObjectPacket.EntityType'
is no longer accessible as an attribute of the the `SpawnObjectPacket'
class: the values now depend on a `ConnectionContext`, and must be
accessed through an instance, or using `SpawnObjectPacket.field_enum'.
See the text of the `AttributeError` raised from the descriptor for
`SpawnObjectPacket.EntityType` for the full details.
(2) For some subclasses of `networking.types.Type', it is necessary to
call the methods `read_with_context' and `send_with_context' instead of
`read' and `send', supplying a `ConnectionContext' for those data types
- currently only `Position` - whose layout depends on it.
