Add support for exporting references.

[cristianoc]

Fixes #237

[cristianoc]

The PR contains examples on how to export a reference as an abstract type, in case the argument type of the reference requires conversion:

// Abstract version of references: works when conversion is required.

module R: {
  [@genType]
  type t('a);
  let get: t('a) => 'a;
  let make: 'a => t('a);
  let set: (t('a), 'a) => unit;
} = {
  type t('a) = ref('a);
  let get = r => r.contents;
  let make = ref;
  let set = (r, v) => r.contents = v;
};

[@genType]
type t('a) = R.t('a);

[@genType]
let get = R.get;

[@gentype]
let make = R.make;

[@genType]
let set = R.set;

type requiresConversion = {x: int};

// Careful: conversion makes a copy and destroys the reference identity.
[@genType]
let destroysRefIdentity = (x: ref(requiresConversion)) => x;

// Using abstract references preserves the identity.
[@genType]
let preserveRefIdentity = (x: R.t(requiresConversion)) => x;