Convert EventSet into class from typedef
The EventSet object was simply typedef-ed to
a std::deque<UnfoldingEvent *>. While this
worked, it caused a lot of the code in tiny-simgrid
to be a bit more difficult to read since it
required the use of a more C-like idiom of passing
`this` as an explicit argument in EvtSetTools.
Effectively, the idea to absorb all of the EvtSetTools
methods into the EventSet class directly. This will
give a more natural C++ (instead of C) look to the
resulting code in the UDPOR algorithm
