Merge: nitdoc: cache `concern_rank`

Improve performance of nitdoc by 60 times in some cases!

Fixes #692, nitdoc on RnR goes from 8 min to 7 sec.

Thanks-to: Jean Privat <jean@pryen.org>

Pull-Request: #695
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>