Ho instantiation (#1204)

Ho instantiation (#1204)

* Higher-order instantiation.

* Add missing files.

* Document inst match generators, make collectHoVarApplyTerms iterative.

* More documentation.

* More documentation.

* More documentation.

* More documentation

* Refactoring, documentation.

* More documentation.

* Fix comment, reference issue.

* More documentation. Fix ho trigger for the changes from master.

* Minor

* More documentation.

* More documentation.

* More

* More documentation, make indexing and lookup in TriggerTrie iterative instead of recursive.

* More

* Minor improvements to comments.

* Remove an unimplemented optimization from an old version of cached multi-trigger matching. Update and improve documentation in inst match generator.

* Improve documentation for ho_trigger.

* Update ho trigger to not access now private member of TermDb.

* Address comments.

* Address

* Clang format