Rewire RLN implementation to use RLN API module

### Background
https://github.com/logos-co/logos-lips/blob/3f7264b90ad8365c68a04506edb7daf03b7c28e8/docs/anoncomms/raw/rln-api.md

### Details
Rewire the current implementation to use the RLN module.


### Acceptance criteria
- All existing RLN tests pass
- remove old implementation (e.g. `group_manager.nim`)
