Resources should be able to return embedded versions of other resources.

To summarize our current thinking:

• Callback can convert its return value to WP_JSON_Response and add HAL-compatible links to other resources, as specified by the route.
• If the request asks for embedded resources, the Server will add embedded representations of the resources to the request by resolving the route (codename Rrrrrrr)
• "embedded" is a context which returns the bare minimum of public data.

At least two open questions:

• If prepare_item_for_response is supposed to add references to other entities, how does that work for collections?
• How can one Response type know how to reference another Response type without knowing its Route? Do we need a global naming paradigm too, or would it be acceptable to reference a Controller class, which would make it easier to look up via the server?