feat(frontends/lean): coercion num -> int even when nat is not open, closes #219

I also had to mark the coercions as reducible.
Otherwise, given the constraint

          ?M (int.of_num 0) =?= (int.of_nat (nat.to_nat 0))

the unifier will not generate the solution

          ?M := fun x, x

library/data/int/basic.lean

@@ -208,7 +208,8 @@ protected opaque definition has_decidable_eq [instance] : decidable_eq ℤ :=
 _
 
 irreducible int
-definition of_nat [coercion] (n : ℕ) : ℤ := psub (pair n 0)
+definition of_nat [coercion] [reducible] (n : ℕ) : ℤ := psub (pair n 0)
+definition of_num [coercion] [reducible] (n : num) : ℤ := of_nat (nat.to_nat n)
 
 theorem eq_zero_intro (n : ℕ) : psub (pair n n) = 0 :=
 have H : rel (pair n n) (pair 0 0), by simp,

library/data/nat/basic.lean

@@ -47,7 +47,7 @@ definition add (x y : ℕ) : ℕ :=
 nat.rec x (λ n r, succ r) y
 infixl `+` := add
 
-definition to_nat [coercion] (n : num) : ℕ :=
+definition to_nat [coercion] [reducible] (n : num) : ℕ :=
 num.rec zero
     (λ n, pos_num.rec (succ zero) (λ n r, r + r + (succ zero)) (λ n r, r + r) n) n