Receive vias don't change the scope of their associated [bus] on @rescope

[bgribble]

This is the leftover work from #200, not critical for the 0.05 release. Trying to change the scope of a
receive-via doesn't work as expected, because the [bus] object will stay in the old scope. There isn't an easy way in a patch to find and target this object for its own @rescope message.

Unfortunately it's not clear what the right semantics are. So... punt!