Hint Resolve unsigned_repr: ints.

(* Specifications: *)
Theorem signed_repr:
forall z, min_signed <= z <= max_signed -> signed (repr z) = z.
Theorem signed_eq_unsigned:
forall x, unsigned x <= max_signed -> signed x = unsigned x.
Theorem signed_positive:
forall x, signed x >= 0 <-> unsigned x <= max_signed.
Theorem signed_repr:
forall z, min_signed <= z <= max_signed -> signed (repr z) = z.
Theorem signed_eq_unsigned:
forall x, unsigned x <= max_signed -> signed x = unsigned x.
Theorem signed_positive:
forall x, signed x >= 0 <-> unsigned x <= max_signed.