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_test.go

@@ -29,7 +29,7 @@ func TestWalkTree(t *testing.T) {
 
 	// start tree walker
 	treeJobs := make(chan restic.WalkTreeJob)
-	go restic.WalkTree(repo, sn.Tree, done, treeJobs)
+	go restic.WalkTree(repo, *sn.Tree, done, treeJobs)
 
 	// start filesystem walker
 	fsJobs := make(chan pipe.Job)