The queue coalesces, so a producer with no consumer never fails: it
leaves one row per subject for ever. That is how 23,582 Thumbnail jobs
accumulated unnoticed (#73), and nothing at runtime would have said so.
every_queued_kind_has_a_consumer reads the shipping sources of every
crate under core/, ui/, apps/ and platform/ (cfg(test) items dropped)
and pairs the JobKind named at each enqueue( call with the kinds named
in a fn kinds( body or a claim_next_matching( call. An enqueue that does
not spell its kind is refused, since the pairing could not be checked.
It guards against passing over nothing: the queue's own files and the
scan must have been read. A second test runs the reader over fixed
snippets so a parsing bug shows up as a failure. Run against master's
scan.rs and walk.rs it names all three orphan enqueues.
Refs #73
