add this lemma to `classical_orders.v` when dropping support for MathComp < 2.3

[affeldt-aist]

Lemma big_lexi_order_prefix_closed_itv {d} {K : nat -> tbOrderType d} n
  (x : big_lexi_order K) :
  same_prefix n x = `[
    (@start_with K n x (fun=>\bot):big_lexi_order K)%O,
    (start_with n x (fun=>\top))%O].
Proof.
rewrite eqEsubset; split=> [z pfxz|z]; first last.
  rewrite set_itvE /= => xbt; apply: same_prefix_trans.
    exact: (start_with_prefix _ (fun=> \bot)%O).
  apply/same_prefix_sym/(big_lexi_order_between xbt).
  apply: same_prefix_trans (start_with_prefix _ _).
  exact/same_prefix_sym/start_with_prefix.
rewrite set_itvE /= !leEbig_lexi_order; apply/andP; split;
  case E : (first_diff _ _) => [m|] //; rewrite /start_with /=.
- by case: ifPn => [/pfxz -> //|]; rewrite le0x.
- by case: ifPn => [/pfxz -> //|]; rewrite lex1.
Qed.

tbOrderType is not yet available in MathComp 2.2

@zstone1