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.

Note that from the lurette point of view, sasa plays the role of the env
as it needs to start playing, to provide Lutin the enable values for
the first demon step.

with algo several actions, and allow lurette to chose them.

In conjunction with -rif, that change makes it possible to use sasa with lurette!

todo: where several actions are enabled in a process, one is chosen at random
at the sasa level (whereas it should be possible at the lutin level).