Revert "remove dead code"

This reverts commit bc65fc56ed.

common/block.go

@@ -385,3 +385,12 @@ func (pb *posBuf) Read(p []byte) (int, error) {
 	pb.Pos += pl
 	return pl, nil
 }
+
+func (pb *posBuf) ReadByte() (byte, error) {
+	if pb.Pos >= len(pb.Data) {
+		return 0, io.EOF
+	}
+	c := pb.Data[pb.Pos]
+	pb.Pos++
+	return c, nil
+}