Dedicated match method

[domn1995]

As a developer, I want a dedicated Match() method, so that consumers of my union type are required to handle each case in order to get a value out of it.

Usage might look something like this:

[Union]
public interface IShape
{
    IShape Circle(double radius);
    IShape Rectangle(double length, double width);
    IShape Triangle(double @base, double height);
}

var shape = new Rectangle(3, 4);
var area = shape.Match(
    circle => 3.14 * circle.Radius * circle.Radius,
    rectangle => rectangle.Length * rectangle.Width,
    triangle => triangle.Base * triangle.Height / 2
);

Console.WriteLine(area); // "12"

[domn1995]

Closed by #7