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.

backend/generic.go

@@ -23,10 +23,7 @@ const hashSize = sha256.Size
 
 // Hash returns the ID for data.
 func Hash(data []byte) ID {
-	h := hashData(data)
-	id := make([]byte, IDSize)
-	copy(id, h[:])
-	return id
+	return hashData(data)
 }
 
 // Find loads the list of all blobs of type t and searches for names which