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.

cmd/restic/integration_fuse_test.go

@@ -147,7 +147,7 @@ func TestMount(t *testing.T) {
 		checkSnapshots(repo, mountpoint, snapshotIDs)
 
 		// third backup, explicit incremental
-		cmdBackup(t, global, []string{env.testdata}, snapshotIDs[0])
+		cmdBackup(t, global, []string{env.testdata}, &snapshotIDs[0])
 		snapshotIDs = cmdList(t, global, "snapshots")
 		Assert(t, len(snapshotIDs) == 3,
 			"expected three snapshots, got %v", snapshotIDs)