The Roslyn-based GenAPI must substitute operator's implementation body throw null

We must implement `CSharpSyntaxRewriter.VisitOperatorDeclaration` to handle operators.
```C#
public class Car : System.IEquatable<Car>
{
///...
    public static bool operator ==(Car lhs, Car rhs) {  } // => { throw null; }
    public static bool operator !=(Car lhs, Car rhs) {  } // => { throw null; }
}
```