Now LayoutZipper can be circular, which means layout switching operates
semantically identically to how it did before making the typesafe
selector.

The selector was very much an acedemic exercise, but since it is working
as expect, I will keep it. I like the type-safety and it can be used as
an example for similar exercises.

So, when doing something like a "zoom", it will only affect the current
layout. Before a "zoom" would be persistent across all layouts, now it
will only occur on the current layout.

I'm still trying to decide if I like it this way or not. It looks like
it'll have minimal impact on things. I don't use different layouts that
often anyway.