Refactor entity associations onto `betty.collections`

[bartfeenstra]

To do

• type on KeyedCollection
• we need event handling support, specifically to act on individual associates being added or removed, to set bidirectionality. We could (maybe not should?) use value resolvers to add new associates. Also we could use the main collection resolver but we'd have to do all the comparisons ourselves.

Blocked by

• Refactor betty.collections #3730