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.

repository/config.go

@@ -70,7 +70,7 @@ func LoadConfig(r JSONUnpackedLoader) (Config, error) {
 		cfg Config
 	)
 
-	err := r.LoadJSONUnpacked(backend.Config, nil, &cfg)
+	err := r.LoadJSONUnpacked(backend.Config, backend.ID{}, &cfg)
 	if err != nil {
 		return Config{}, err
 	}