Support using calculations as relationship keys

At the moment, if you want to do something like this you need to use a manual relationship. Ideally, we'd be able to support using calculations as relationship keys out of the box, allowing for complex relationships to be defined easily. However, these naturally would not be able to be used with `manage_relationship`.

The issue names no files or tests. Start by tracing how manual relationships, relationship keys, calculations, and manage_relationship are defined; done means calculations can serve as relationship keys while remaining unsupported by manage_relationship.

Written by the indexing model from the issue text.