Add type blobref.SizedBlobRef

lib/go/blobref/blobref.go

@@ -32,11 +32,18 @@ var supportedDigests = map[string]func()hash.Hash{
 	},
 }
 
+// BlobRef is an immutable reference to a blob.
 type BlobRef struct {
 	hashName  string
 	digest    string
 }
 
+// SizedBlobRef is like a BlobRef but includes a (potentially mutable) size.
+type SizedBlobRef struct {
+	*BlobRef
+	Size int64
+}
+
 type ReadSeekCloser interface {
 	io.Reader
 	io.Seeker