Summary:
For navigation actions at high level, reducers from NavigationReducers does not
know anything about the app-specific state thus people won't use these reducers.
Instead, people should build their own reducers.
There are a lot of good libraries available that help people to reducing things if that's
what they really need.
At the low level, for navigation state changes that don't involve app-specific state,
`NavigationStateUtils` should server that kind of need.
`NavigationReducers` serves little benefit cause it does not know the app state, it does
not know how to traverse the navigation states which can be a tree, a list or a map.
That said, we hold no interest in owning in the core navigation library.
Reviewed By: ericvicenti
Differential Revision: D3372910
fbshipit-source-id: 797382b46e7d64b7ad578b51dd37e2b941faa83d
3a8b50ad55