support polymorphic ocaml.type

[ygrek]

With #10 fixed, consider the following :

$ cat a.proto 
type q 'a = [ 'a ] options "ocaml.type" = "'a list, f, g"
type iq = { i : q<int>; }

$ extprotc a.proto -o /dev/stdout
module Q =
  struct type 'a q = 'a list;; let pp_q pp_a ppf x = Extprot.Pretty_print.pp_list pp_a ppf (g x);; 
  end;;
module Iq =
  struct
    type iq = { i : 'a list };;
    let pp_iq =
      Extprot.Pretty_print.pp_struct
        [ ("Iq.i",
           (Extprot.Pretty_print.pp_field (fun t -> t.i) (Q.pp_q Extprot.Pretty_print.pp_int))) ];;

  end;;

type iq is wrong, afaiu extprot doesn't substitute type variables in ocaml.type - fair enough, but it seems an easy fix would be to generate type iq = { i : int q; } ie going through left-hand type. Sorry if my understanding is too naive.

[ygrek]

Disregard, this obviously doesn't hold when ocaml.type is different from original type (which is 99% of non-trivial cases)