Add another level of modal operators

We would like to be able to use modal operators not just on individual atoms, but on rules as a whole. For example, we would like to be able to write the following to represent the insertion of a rule (which itself uses a modal) into the policy named "classification".

insert[execute[p(x)] :- q(x), "classification"]

This involves (i) modifying the grammar to enable inserts/deletes of rules, and (ii) modifying the code that loads a parsed formula to create an Event object. We probably also want to add a type-checker like is_event (similar to the is_datalog, is_atom functions found in compile.py).

This functionality allows the 'simulate' API call to include additions/deletions of rules with modals.