graph: reference/referrer node types work with graph traversal.

graph: reference/referrer node types work with graph traversal.

The graph traversal procedures in (guix graph) assume that nodes can be
compared with 'eq?', which was not the case for nodes of
%REFERENCE-NODE-TYPE and %REFERRER-NODE-TYPE (strings).

* guix/scripts/graph.scm (intern): New procedure.
(ensure-store-items, references*)
(%reference-node-type, non-derivation-referrers)
(%referrer-node-type): Use it on all store items.
* tests/graph.scm ("node-transitive-edges, references"): New test.