#22 Idempotent event implementation. 'once' needs more internal refactoring as edts reference events guid long after they are satisfied. We need to overhaul a bit the dependence mecanism to avoid that.
Fixes #22. Implements ONCE events. Instead of carrying event's guid in the edt, we directly store the data guid events have been satisfied with. Once-events can then safely be deallocated since the runtime doesn't hold any guid reference to them.
Fixes #18. Now plays nicely with finish-edt.