SameAs should warn if used with value types

[jnm2]

Motivation: nunit/nunit#3614 (comment)

If either the expected value or actual value or both are a value-typed expression, Assert.AreSame or Is.SameAs are guaranteed to fail because C# will implicitly convert the value type to a new boxed instance. It doesn't make sense to use AreSame/SameAs unless the types of the passed expressions are both reference types.

It would make sense to offer a fix to change SameAs to EqualTo since that was probably the intent.

Should show an error or warning on SameAs:

class C
{
    void M()
    {
        var x = 3;
        Assert.That(x, Is.SameAs(3));

        // Also this
        Assert.That((object)x, Is.SameAs(3));

        // Also this
        Assert.That(x, Is.SameAs((object)3));
    }
}

Should not show an error or warning because the expressions are both reference-typed:

class C
{
    void M()
    {
        var x = 3;
        Assert.That((object)x, Is.SameAs((object)3));
    }
}

[manfred-brands]

I will work on this over the weekend.

[manfred-brands]

Analyzer and UnitTest done. Code Fix pending.

[manfred-brands]

Code Fix and documentation done.