#1148 Keep every Draft 3 graph form edge inside the document

An edge of the graph stood for any subschema that was nothing but a reference, including one whose target lived in another document. Such an edge could not reach an entry, so the reference it carried had to be written back into every body that reached it, and the form admitted edges that were not edges of the graph at all.

An edge is now only a reference that stays in the document. One that leaves it becomes an entry of its own, so every edge points at #/definitions/<integer> and the special case for carrying a foreign URI back into the bodies goes away.

The metaschema says so too: entry and edge had the same shape and are now one definition, and definitions must hold at least one entry, since the wrapper always points at one.