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.

pack/pack_test.go

@@ -31,7 +31,7 @@ func TestCreatePack(t *testing.T) {
 		_, err := io.ReadFull(rand.Reader, b)
 		OK(t, err)
 		h := sha256.Sum256(b)
-		bufs = append(bufs, Buf{data: b, id: h[:]})
+		bufs = append(bufs, Buf{data: b, id: h})
 	}
 
 	file := bytes.NewBuffer(nil)