Add semantic equivalence to UnfoldingEvent

Add semantic equivalence to UnfoldingEvent

Two UnfoldingEvents are considered to be equivalent
if they have the same associated action (same actor,
type, and times_considered) and the same immediate history.
Semantic equivalence is required since any given
event may appear in the extension set of several configurations
that UDPOR decides to explore, and thus we must be able
to determine if a newly-computed event has already been
seen before