Add workaround for subtlety with state regeneration

Add workaround for subtlety with state regeneration

Regeneration of the Execution inside DFSExplorer
should be as simple as choosing the prefix relative
to the appropriate backtracking point. However,
it did not appear to function so simple. After
a good amount of debugging, it appears that
the stack contents can change unexpectedly so
(e.g. a transition "in the middle" of the stack
seemingly switches arbitrarily). Until we can
pinpoint the true cause here, we simple resort
to rebuilding the execution based off the new
stack each time we decide to backtrack. The
downside is that all clock vectors have to be
recomputed after each backtrack. For now,
this is something that's OK to live with,
especially considering that this certainly
won't be a bottle neck in performance