[WIP] Split Referrers into MixedReferrers and Referrers with cascading

this replaces #234 

we now have two distinct mappings:

* MixedReferrers behaves like the Referrers we had, but has no filter attribute. It is an immutable collection of any referrers to this document
* Referrers is a fully mapped field like ReferenceMany where you can add and remove items. instead of `filter` the attribute is properly named `mappedBy` and required.