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/cmd_cat.go

@@ -54,8 +54,6 @@ func (cmd CmdCat) Execute(args []string) error {
 	if tpe != "masterkey" && tpe != "config" {
 		id, err = backend.ParseID(args[1])
 		if err != nil {
-			id = nil
-
 			if tpe != "snapshot" {
 				return err
 			}