round method
Returns the integer value closest to this num.
Rounds away from zero when there is no closest integer:
(3.5).round() == 4 and (-3.5).round() == -4.
Implementation
Rational round() {
final abs = this.abs();
final absBy10 = abs * _r10;
var r = abs.truncate();
if (absBy10 % _r10 >= _r5) r += _r1;
return isNegative ? -r : r;
}