Remove explicit generator (#1962)

- Remove the old, multi-target `event_generator` class in favour of `schedule_generator(tgt, weight, explicit_schedule)`
- Much simplification ensues, `event_generator` is no longer a type-erasing container, but just what 
   `schedule_generator` was before
- Make the label resolution in generators a bit more eager, no longer at simulation time, 
   but now during setup (bit give a wee bit of perf as well)

Closes #1488