Reimplement support for relational triggers (#7063)

Reimplement support for relational triggers (#7063)

This makes relational triggers its own kind of instantiation match generator. Future PRs will delete the code interleaved in the core InstMatchGenerator class for handling relational triggers.

This PR also fixes two issues related to how trigger techniques are combined:
(1) instantiation match generators that are generated as part of multi-triggers now are properly specialized (using getInstMatchGenerator),
(2) there was a bug with auto-generated triggers: when the first auto-generated trigger was generated that was already a user trigger, then we would ignore all other auto-generated triggers.

This is work towards finding a solution for the choice.move.hard Facebook benchmark, where relational-triggers appear to be a possible solution.