Move Router to stack sub-package and rename it to StackRouter

[arkivanov]

Router organizes children as a stack - there is always one resumed child on top of the stack, children in the back stack are either stopped or destroyed. In the future, there could be other kinds of routers, e.g. a router that organizes children as a list (all children are active, no back stack).

To allow the room for additional routers, it is necessary to move Router and all its surroundings to stack sub-package. And also rename it to StackRouter - to make it clear that it acts as a stack. It also makes sense to rename RouterState to ChildStack.

This change should be done via deprecation cycle.