* Refactor, move child navigation to navigation prop
This fixes our descriptor caching issue, and unblocks explicit nested navigation options.
As a side effect, the following APIs are introduced:
- navigation.getChildNavigation(routeKey) , which is useful for explicitly getting children config info
- navigation.router, access to the static router
- router.childRouters[routeName] , an optional way to access the children routers directly. If childRouters are not provided in a router, we will fall back on getComponentForRouteName(routeName).router, which is the previous external API for this (although it may be slower because it will require the whole screen component).
* supporting tests and top level actions
* cleanup
