Restructure Container and BindingBuilder to be unidirectional

[mscharley]

The only other structural thing that jumped out at me is that I don't love the Container/BindingBuilder bidirectionally referencing each other and storing the in-progress binding state outside the container. I'd move the registry inside the container and pass an addBinding callback into the BindingBuilder so the Container is a full, authoritative source of truth about its own state.

/via @ian-moriarty-equiem