refactor(set_theory/game/ordinal): Improve API (#13878)
refactor(set_theory/game/ordinal): Improve API (#13878)

We change our former equivalence `o.out.α ≃ o.to_pgame.left_moves` to an equivalence `{o' // o' < o} ≃ o.to_pgame.left_moves`. This makes two proofs much simpler. 

We also add a simple missing lemma, `to_pgame_equiv_iff`.



Co-authored-by: Scott Morrison <63f04b4d96235eb042cc46d94f14d241d2a26722@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib