Allow void as covariance generic argument

[ds1709]

Currently in C# void can not be used as generic type argument. But there are scenarios when it can be usefull. E.g.:

public interface IFoo<out T>
{
    T DoWork();
}

public sealed class Int32Foo : IFoo<int>
{
    public int DoWork()
    {
        return 0;
    }
}

public sealed class VoidFoo : IFoo<void> // not allowed
{
    public void DoWork()
    {        
    }
}

// somw kind of workaround

public sealed class ObjectFoo : IFoo<object>
{
    public object DoWork()
    {        
    }
}

AFAIK, currently in runtime it's supported to return void type values. More then, it may be necessary in some scenarios when you are working with System.Linq.Expression.
So, it may be usefull to be able to use void as covariance generic type argument in ihnterfaces, where it used as return type of methods.

[HaloFour]

See: #696