Move the Walk types into registry/storage/driver, and add a Walk method to each
storage driver. Although this is yet another API to implement, there is a fall
back implementation that relies on List and Stat. For some filesystems this is
very slow.
Also, this WalkDir Method conforms better do a traditional WalkDir (a la filepath).
This change is in preparation for refactoring.
Signed-off-by: Sargun Dhillon <sargun@sargun.me>
