Allows no printer for parameters

[vch9]

(** ['a spec] combines an ['a gen] and a printer *)
type 'a spec = { gen : 'a gen; printer : 'a printer option}

(** [t] is the specification type, describing a function.
    Thus [t] declaration must end with {! (^>>) }. *)
type ('fn, 'r) t =
  | Result : 'a printer -> ('a, 'a) t
  | Arrow : 'a spec * ('fn, 'r) t -> ('a -> 'fn, 'r) t

Parameters should not be printed. Since new snapshots will use previous
parameters, we know that they are equals, it's not mandatory to print them.
However, the result printer must remains.

[vch9]

However, #10 should be done before this issue. We don't want to have a difference with new snapshot includes new printers