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.

tree.go

@@ -94,7 +94,7 @@ func (t Tree) Find(name string) (*Node, error) {
 func (t Tree) Subtrees() (trees backend.IDs) {
 	for _, node := range t.Nodes {
 		if node.Type == "dir" && node.Subtree != nil {
-			trees = append(trees, node.Subtree)
+			trees = append(trees, *node.Subtree)
 		}
 	}