Add fraction support.
Supported platform: NetCore 1, NetCore 2, NetFx 4, NetStd 1.3, NetStd 2.0
Using an instance built by default parameterless constructor will cause exception or miscalculation. To avoid this, always use parameter-based constructors. This will not be fixed due to consideration about running speed.
Fraction a = new Fraction(2, 3); // 2/3
Fraction b = 100; // 100
Fraction c = new Fraction("50 1/2"); //50 1/2
Fraction d = a + b;