Store .dep files as bitmasks
Imported issue: Initially reported by Alexander Kruppa in https://gforge.inria.fr/tracker/?group_id=2065&aid=17136
Each .dep file stores, for one dependency, which relations are included in that dependency.
Currently, each .dep file stores the list of a,b-coordinates of included relations. This wastes a lot of space: with 20 .dep files, and if about half of the relations are included in a given dependency, we store 20*.5 = 10 times the full list of a,b coordinates across different files. These files make up on the order of 10% of the total disk space for a factorization (about half that when compressed).
Instead, we could have one file with all a,b-pairs, and in each .dep file store a bit-mask of which of these a,b-pairs belong to this dependency.