fix: track rule transitions in the state struct

This commit increases the size of the state struct by 8 bytes -- in checkers
with counterexample traces enabled -- and uses this new member to print the
transition rules in counterexample traces. The purpose of this is to side step
a problem wherein trying to find the transition rule between two states actually
*re-triggers* a checker error.

Interestingly this case that recurses was not detected with the existing test
suite because examples that trigger it simply take an exit as if they were a
subordinate thread. The fact that print_transition never properly returns was
uncovered when I tested the machine readable output on tests/error-statement.m.
This example produced invalid XML because it never closed the "<error>" tag.
This issue should now be resolved.

As an aside, debugging this made me think the way CMurphi constructs
counterexample traces is inherently wrong. I don't think it can correctly handle
assertion and error failures (as opposed to invariants), but the problems are
masked for similar reasons to the ones I describe above. I think extending
CMurphi to add support for more than one error before terminating would
encounter this issue.