kore-exec should output a count of unexplored leaves when it terminates

[jberthold]

When a proof fails, kore-exec will output a counterexample (or more than one, with --max-counterexamples). However, the output of counterexamples does not provide information about how many branches/leaves remained unexplored and might therefore yield additional counterexamples.

This can be found out in debugger mode by calling leafs.
The CLI could output this information when kore-exec terminates.

Needs #3143

[ana-pantilie]

@jberthold Let's timebox this: let me know at the end of your day tomorrow what your progress has been and we'll decide what to do with it.

[jberthold]

Status: This feature required modifications to how the execution graph is explored (leavesM in Kore.Rewrite.Strategy) and instigated a rewrite of the graph traversal functionality that leavesM provides. Code is in #3163 (closed for archiving), and will be cleaned up so it can be merged and used for kore-exec.