Refactor checkInt and checkBigInt

[leonardocustodio]

Refactor checkInt and checkBigInt

[gabrielokura]

/// Returns `true` if the val is `num` and is in range of [min, max] with `min` and `max` being inclusive.
/// Otherwise, returns `false`.
/// The [value] argument must be a value that is comparable to
/// [min] and [max] objects so the method won't fail in run-time.
/// See also: [Comparable]
bool compare(Comparable value, Comparable min, Comparable max) {
  return value.compareTo(min) >= 0 && value.compareTo(max) <= 0;
}

[justkawal]

Changed approach in #186