All the get_* functions (that are of no interest for users) are now in
the Register module.

But I bumped into the "value restriction" limitation
https://stackoverflow.com/questions/22507448/the-value-restriction

Indeed, I need to store functions of 'v in some tables (or references),

And tables are necessaryly weakly polymorphic !

https://v1.realworldocaml.org/v1/en/html/imperative-programming-1.html#side-effects-and-weak-polymorphism

Save/restore the PRGS at checkpoints (necessary for internal demons).

Rationale: it was not really necessary (Algo.neighbor contains almost
the same information), and the Topology.neighbor type was alone in its
module, which looked weird.

For some reason, I believed that the source dir containing the main
program need to be named bin for dune to work smoothly. But actually,
it seems to work fine with src, so let's go for it.