Introduce Walk Method Per Storage Driver

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>

registry/storage/driver/inmemory/driver.go

@@ -240,6 +240,12 @@ func (d *driver) URLFor(ctx context.Context, path string, options map[string]int
 	return "", storagedriver.ErrUnsupportedMethod{}
 }
 
+// Walk traverses a filesystem defined within driver, starting
+// from the given path, calling f on each file
+func (d *driver) Walk(ctx context.Context, path string, f storagedriver.WalkFn) error {
+	return storagedriver.WalkFallback(ctx, d, path, f)
+}
+
 type writer struct {
 	d         *driver
 	f         *file