hashCode should be stable for any scale

[a14n]

  var a = BigDecimal.parse('1.000');
  var b = BigDecimal.parse('1.00');
  // a and b are equal, but they have different hash codes
  print(a.hashCode);
  print(b.hashCode);
  print(a.hashCode == b.hashCode); // false (SHOULD be true)
  print(a == b); // true
  // so a set containing `a` does not contain `b` although they are equal
  print({a}.contains(b)); // false (SHOULD be true)