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

Things that are currently broken:
- the ability to init vars in the dot
- controling the name of local vars in the rif output

The idea is to use the Obj module (arg !!!), which is safe thanks to
the fact that there is now only one register function, which forces all the
'v to be bound to the same type.

Indeed, by using the Env.nset and Env.nget defined in
0d6575a7 a pb that was hided by the
use of Env.set and Env.get was triggered.

The bug was that variables initialisations were done several times.

In sasa.ml, the way I was updating the Env.t by iterating on the
variables was forcing to "close" expressions that were calling init
functions.

The rationale is that
 - it is not always the cmxs that is necessary .e.g., rdbg uses .cma or .ml
 - it makes more sense to refer to the source code

instead of duplicating its content

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.