feat: add Int.le_add_one_iff (#9892)

Co-authored-by: Yury G. Kudryashov <urkud@urkud.name>

Mathlib/Data/Int/Order/Basic.lean

@@ -136,6 +136,9 @@ theorem pred_self_lt (a : ℤ) : pred a < a :=
 #align int.lt_add_one_iff Int.lt_add_one_iff
 #align int.le_add_one Int.le_add_one
 
+theorem le_add_one_iff {m n : ℤ} : m ≤ n + 1 ↔ m ≤ n ∨ m = n + 1 := by
+  rw [le_iff_lt_or_eq, lt_add_one_iff]
+
 theorem sub_one_lt_iff {a b : ℤ} : a - 1 < b ↔ a ≤ b :=
   sub_lt_iff_lt_add.trans lt_add_one_iff
 #align int.sub_one_lt_iff Int.sub_one_lt_iff
@@ -145,13 +148,8 @@ theorem le_sub_one_iff {a b : ℤ} : a ≤ b - 1 ↔ a < b :=
 #align int.le_sub_one_iff Int.le_sub_one_iff
 
 @[simp]
-theorem abs_lt_one_iff {a : ℤ} : |a| < 1 ↔ a = 0 :=
-  ⟨fun a0 => by
-    let ⟨hn, hp⟩ := abs_lt.mp a0
-    rw [← zero_add 1, lt_add_one_iff] at hp
-    -- Defeq abuse: `hn : -1 < a` but should be `hn : 0 λ a`.
-    exact hp.antisymm hn,
-    fun a0 => (abs_eq_zero.mpr a0).le.trans_lt zero_lt_one⟩
+theorem abs_lt_one_iff {a : ℤ} : |a| < 1 ↔ a = 0 := by
+  rw [← zero_add 1, lt_add_one_iff, abs_nonpos_iff]
 #align int.abs_lt_one_iff Int.abs_lt_one_iff
 
 theorem abs_le_one_iff {a : ℤ} : |a| ≤ 1 ↔ a = 0 ∨ a = 1 ∨ a = -1 := by

Mathlib/Data/Nat/Order/Basic.lean

@@ -181,12 +181,8 @@ theorem add_eq_three_iff :
     ← add_assoc, succ_inj', add_eq_two_iff]
 #align nat.add_eq_three_iff Nat.add_eq_three_iff
 
-theorem le_add_one_iff : m ≤ n + 1 ↔ m ≤ n ∨ m = n + 1 :=
-  ⟨fun h =>
-    match Nat.eq_or_lt_of_le h with
-    | Or.inl h => Or.inr h
-    | Or.inr h => Or.inl <| Nat.le_of_succ_le_succ h,
-    Or.rec (fun h => le_trans h <| Nat.le_add_right _ _) le_of_eq⟩
+theorem le_add_one_iff : m ≤ n + 1 ↔ m ≤ n ∨ m = n + 1 := by
+  rw [le_iff_lt_or_eq, lt_add_one_iff]
 #align nat.le_add_one_iff Nat.le_add_one_iff
 
 theorem le_and_le_add_one_iff : n ≤ m ∧ m ≤ n + 1 ↔ m = n ∨ m = n + 1 := by