Match any values for ref parameters using It.Ref<TValue>.IsAny / ItExpr.Ref<TValue>.IsAny

[stakx]

This PR adds It.Ref<T>.IsAny which is to ref parameters what It.IsAny<T>() is to regular by-value parameters:

interface IFoo
{
    void Do(ref int n);
}

var mock = new Mock<IFoo>();
mock.Setup(m => m.Do(ref It.Ref<int>.IsAny));

This resolves #479.