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_find.go

@@ -114,7 +114,7 @@ func (c CmdFind) findInSnapshot(repo *repository.Repository, id backend.ID) erro
 		return err
 	}
 
-	results, err := c.findInTree(repo, sn.Tree, "")
+	results, err := c.findInTree(repo, *sn.Tree, "")
 	if err != nil {
 		return err
 	}