message: Loosen ProductId prefix constraints.
  I've tested this locally against the Stripe API. This works for custom
  product IDs & works for product IDs generated by Stripe.
  Closes
diff: @@ -501,7 +501,7 @@ def_id!(
  );
  def_id!(PersonId, "person_");
  def_id!(PlanId: String); // N.B. A plan id can be user-provided so can be any arbitrary string
  -def_id!(ProductId, "prod_");
  +def_id!(ProductId: String);
  def_id!(RecipientId: String); // FIXME: This doesn't seem to be documented yet
  def_id!(RefundId, "re_");
  def_id!(ReviewId, "prv_");
  