Make use of the sets `G` and `U` with `Unfolding`
The `Unfolding` class in UDPOR previously discarded
all events from `U` when they were no longer needed.
Technically, UDPOR moves those events into a set `G`
which can be used to do more interesting things
in the future (e.g. to model check cyclic state spaces).
Furthermore, there are some subtlties it seems in
fully destroying the contents of an event after it has
been discovered, viz. with dealing with what to do when
a previous extension set contains such a deleted event
