Use array instead of hash for backend.ID

Since backend.ID is always a slice of constant length, use an array
instead of a slice. Mostly, arrays behave as slices, except that an
array cannot be nil, so use `*backend.ID` insteaf of `backend.ID` in
places where the absence of an ID is possible (e.g. for the Subtree of a
Node, which may not present when the node is a file node).

This change allows to directly use backend.ID as the the key for a map,
so that arbitrary data structures (e.g. a Set implemented as a
map[backend.ID]struct{}) can easily be formed.

walk.go

@@ -32,7 +32,7 @@ func walkTree(repo *repository.Repository, path string, treeID backend.ID, done
 	for _, node := range t.Nodes {
 		p := filepath.Join(path, node.Name)
 		if node.Type == "dir" {
-			walkTree(repo, p, node.Subtree, done, jobCh)
+			walkTree(repo, p, *node.Subtree, done, jobCh)
 		} else {
 			select {
 			case jobCh <- WalkTreeJob{Path: p, Node: node}: