Extend Entity relations: multiple per Entity

[mlange-42]

Upcoing v0.8.0 features entity relations (see milestone 1).
However, each entity can only have one relation to a single other entity.

This issue is for collecting thoughts on extending to multiple relations per entity.

Required

• [ ] Map-like data structure for mapping multiple entities (targets) to a node archetype, including wildcard/nil
• [ ] Filters that support multiple relation targets