resources: Implement Send -> Recv

This is a new design idea which I had. Whether it stays around or not is
up for debate. For now it's a rough POC.

The idea is that any resource can _produce_ data, and any resource can
_consume_ data. This is what we call send and recv. By linking the two
together, data can be passed directly between resources, which will
maximize code re-use, and allow for some interesting logical graphs.

For example, you might have an HTTP resource which puts its output in a
particular file. This avoids having to overload the HTTP resource with
all of the special behaviours of the File resource.

For our POC, I implemented a `password` resource which generates a
random string which can then be passed to a receiver such as a file. At
this point the password resource isn't recommended for sensitive
applications because it caches the password as plain text.

Still to do:
* Statically check all of the type matching before we run the graph
* Verify that our autogrouping works correctly around this feature
* Verify that appropriate edges exist between send->recv pairs
* Label the password as generated instead of storing the plain text
* Consider moving password logic from Init() to CheckApply()
* Consider combining multiple send values (list?) into a single receiver
* Consider intermediary transformation nodes for value combining

pgraph/pgraph.go

@@ -659,6 +659,14 @@ func (g *Graph) Process(v *Vertex) error {
 		}
 
 		obj.SetState(resources.ResStateCheckApply)
+
+		// connect any senders to receivers and detect if values changed
+		if changed, err := obj.SendRecv(obj); err != nil {
+			return errwrap.Wrapf(err, "could not SendRecv in Process")
+		} else if changed {
+			obj.StateOK(false) // invalidate cache
+		}
+
 		// if this fails, don't UpdateTimestamp()
 		checkok, err := obj.CheckApply(!obj.Meta().Noop)
 		if checkok && err != nil { // should never return this way