Replace didPushNext() and didReplaceNext() with didChangeNext(), and
call it in more cases, so that a route can easily track the next route.

Use this to make TransitionRoute properly track its next route so that
you can do next-route-driven animations that work even with removes,
replaces, and other crazy manipulations of the navigator stack.

To make it easier to avoid pushing twice in one frame, provide a
transaction mechanism for the navigator.

* NavigatorState.replaceRouteBefore()
* NavigatorState.removeRouteBefore()
* NavigatorState.popUntil()

This lets you dynamically replace one route with another.