This removes the mostly obsolete 'numIn/Out' range restrictions on nodes.
It also makes it possible to connect optional crossbars that disappear.
val x = TLXbar()
x := master
slave := x
val y = TLXbar()
x :=* y // only connect y if it gets used
This will create crossbar x, but crossbar y will disappear.
FlipRendering { implicit p => ... } now changes the render direction of edges.
diplomatic NodeImps can specify a default render flip using the new 'render' method.
We used to place Monitors at the point of the ':='.
This was problematic because the clock domain might be wrong.
Thus, we needed to shove Monitors a lot.
Furthermore, now that we have cross-module ':=', you might not even
have access to the wires at the point where ':=' is invoked.
