Add class for eventual "happens-before" computations
Computing the "happens-before" relation over
a sequence of executions is an important step
in determining whether or not DPOR (and SDPOR
alike) needs to add a backtracking point.
This commit introduces the "ClockVector"
class which acts effectively as a light
wrapping around an `unordered_map` mapping
actor ids to integer values.
The essential component of the ClockVector
class is that its contents are "implicit"
in the sense that actors for which no
value is explicitly contained in the map
are assigned to the value `0` by default.
This allows clock vectors to be flexible
enough to support the creation of new
actors and the enabling/disabling of old
actors as they come and go.
