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.

and reuse it for Neighbor, which make the naming more consistent.

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).

Also add the list of neighbors to the init function. The rationale is that
to give a sensible initial value to local vars of type 'Neighbor', we need
the neighbors number.

I could have chosen to pass an integer only. Well, who can the plus
can the minus.