Some style change & bugfixes

- Changed most of the things that had to to with edge rank and RangeSampler to 64-bit integers
- Moved random functions in their namespace
- Started using XorShift as a RNG (it generates 64bit-long integers)
- Aesthetic changes: indentation & comment style
- Renamed (public) add_random_edges to add_edges and simplified the private one
- Changed adj_list from an array of sets to a single set (and simplified things that used it)