Problem/Motivation
As explained in Proposing an alternative to application/vnd.drupal.ld+json, the new plan is to use HAL as the primary format for REST. First we'll need to decide how to serialize an entity to hal+json.
We will definitely add a link for each entity reference. Some have stated a preference for additionally including the entity reference field with the other fields in the resource's state.
Proposed resolution
Add a set of Normalizers and an Encoder.