engine, lang: Modern exported resources

I've been waiting to write this patch for a long time. I firmly believe
that the idea of "exported resources" was truly a brilliant one, but
which was never even properly understood by its original inventors! This
patch set aims to show how it should have been done.

The main differences are:

* Real-time modelling, since "once per run" makes no sense.
* Filter with code/functions not language syntax.
* Directed exporting to limit the intended recipients.

The next step is to add more "World" reading and filtering functions to
make it easy and expressive to make your selection of resources to
collect!

engine/graph/state.go

@@ -430,3 +430,13 @@ func (obj *State) poll(ctx context.Context, interval uint32) error {
 		obj.init.Event() // notify engine of an event (this can block)
 	}
 }
+
+// hidden is a replacement for Watch when the Hidden metaparameter is used.
+func (obj *State) hidden(ctx context.Context) error {
+	obj.init.Running() // when started, notify engine that we're running
+
+	select {
+	case <-ctx.Done(): // signal for shutdown request
+		return nil
+	}
+}