Add IBelongToARepository

As discussed in #574 and #665, here's an interface that allows access to an instance's corresponding `IRepository` for certain (non-standard) usage patterns.

TODO:

* [ ] Add a meta test that the interface is only ever explicitly implemented
So it does, though this ctor was never meant for public consumption. I've marked this one as `[Obsolete]` in favor of an `internal` one with the new signature.