Basically, the idea is to parse the result of the registred ('s ->
string) function.

For instance if  the user define 'v and 'v printer as follows:

 type state = { a : int ; b : float }
 let state_to_string s = Printf.sprintf "x=%d y=%f" s.v1 s.v2

pid_x and pid_y will be used as output var names in the RIF outputs.

If the user provide no name:
 let state_to_string s = Printf.sprintf "%d %f" s.v1 s.v2

some names will be invented (pid_v0 and pid_v1)

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.

Test: add enable rule so that step always do something (equivalent, but more in the spirit of the model)