test and document co-dependent classes

[brettmc]

• Test co-dependent classes, A::createB() and B::createA().
• Add a test to demonstrate how it works.
• Document

[brettmc]

This method of implementing co-dependent classes is pretty clunky, and mostly to illustrate the problem. I will try to come up with a more ergonomic solution. It's useful to know that it can be done now (even without the macro, I just added that to simplify the usage)
edit: the no-macro version is not actually that bad: 7c2813c

[jmjoy]

You can just write:

const A_CLS: &str = r"IntegrationTest\Dependency\A";
const B_CLS: &str = r"IntegrationTest\Dependency\B";
// Build both class entities
let mut a_cls = ClassEntity::new(A_CLS);
let mut b_cls = ClassEntity::new(B_CLS);

let a_bound_class = a_class.bound_class();
let b_bound_class = b_class.bound_class();