Allow result's extraction of snapshot about osnap

[vch9]

This is not that easy. The Spec must respect the precedent snapshot, thus, it will
not be complicated to well-typed the code with a new f and an old spec.

For instance with the addition:

let pos_add x y =
 let z = x + y in
 if z > 0 then z else -1

let pos_add_new x y =
 let z = x + y in
 if z > 0 then Some z else None

The user can write:

let f x y =
  match pos_add_new x y with
  | Some z -> z
  | None -> -1

let test = Test.(make ~count:5 ~rand ~path ~name:"add" ~spec f) in

Having the extract parameter would not provide a really useful feature.