Makes BackStackScreen<*> practical

[rjrjr]

Adds a few crucial out declarations to various Container and Wrapper types, so that declaring RenderingT types like BackStackScreen<*> is practical. Also required moving fun BackStackScreen.plus out to be an extension function.

Without these changes, this does not compile:

    val foo = BackStackScreen(FooScreen(1))
    val bar = BackStackScreen(BarScreen(1))
    val both = foo + bar

Instead we were forced to do things like:

    val foo: BackStackScreen<Screen> = BackStackScreen(FooScreen(1))
    val bar: BackStackScreen<Screen> = BackStackScreen(BarScreen(1))
    val both = foo + bar

It gets really old really fast.