Before, the `DiskII` object took care of both nibble disks and WOZ
disks at the same time. This made it hard to see which code affected
which disks.
Now, nibble disks are handled by the `NibbleDiskDriver` and WOZ disks
are handled by the `WozDiskDriver`. This separation of code should
lead to easeir testing and, perhaps, faster evolution of disk
handling.
An upcoming change will move the `NibbleDiskDriver` and the
`WozDiskDriver` to separate files.
This passes all tests, compiles, and runs both nibble disks and WOZ
disks.
51ba03ac28