Deriver: remove `merlin.hide` for derived generators

[vch9]

@tomsib2001:

Should merlin know a value gen_toto or toto : arbitrary after the above definition?

Yes.

[vch9]

Actually, I wonder if we need to remove merlin.hide:

module A = struct
  type t = int [@@deriving qcheck]
end

Merlin knows about Q.gen