Allow changing layouts?

At the moment, the library doesn't necessarily care about a specific layout and doesn't explicitly assume a constant layout.

However, I'm not sure what happens if you currently attempt to do this. I suspect the data source would throw an exception if your new layout is radically different and uses different cell classes.

Collection View allows you to animate transitions to another layout:
- https://developer.apple.com/documentation/uikit/uicollectionview#1657143

Plus, delegate methods:
- https://developer.apple.com/documentation/uikit/uicollectionviewdelegate#1656914

In order to accommodate this, we'd have to:

1. update the view model (and cell registrations), but not perform a diff/reload yet
2. update the layout
3. reload?