Adds getNextBackstack

Not ready to merge, just posted for discussion. Being able to ask the flow what's going on during a transition lets us throw away a ton of contorted code that accomplishes exactly the same thing.

This allows the world to see what the stack will look like when the current transition is done.
The listener gets it, but no one else does. Sub- sub- sub- containers that are being displayed due do code way up (down?) the call stack during the Listener#go call need a way to figure out what new screen is coming in to do their job. Some apps (like, say, yours, mine, and every useful one) go through pretty incredible gymnastics to do this themselves.