Rename the base Entity type to ApiEntity, and the base EntityToombstone
to ApiTombstone to better reflect the reality that its only used in the
API interface and that the client and server types uses its own base if
any.
Remove EntityLiving and pull EntityTombstone out of of the base entity
type so that the types based on ApiEntity are always living entities and
if it's possible for it to contain tombstone this will be explicitly
told with the type including a union with ApiTombstone.
Refactor the types of the ClientEntity and ClientMap to better reflect
the types of the entities it stores and converts to/from.
