tmp: Add EXPENSIVE_CHECKS wrapper around fsm_exec's is_isdfa check.

Normally this doesn't matter, because the expected API use is based on
code generation rather than heavily using `fsm_exec`, but we are
calling fsm_exec to check that the combined DFAs still match all of
their original regexes. This is very expensive. (It similarly gets
very cumbersome during fuzzing.)

A better way to address this long-term is tracking on the FSM whether
it's result of fsm_determinise (and optionally fsm_minimise), and
updating that info to indicate that it reverted to an NFA if any
other operations modify the FSM states/edges. That's outside the scope
of this PR though.

src/libfsm/exec.c

@@ -105,10 +105,12 @@ fsm_exec(const struct fsm *fsm,
 
 	/* TODO: pass struct of callbacks to call during each event; transitions etc */
 
+#if EXPENSIVE_CHECKS
 	if (!fsm_all(fsm, fsm_isdfa)) {
 		errno = EINVAL;
 		return -1;
 	}
+#endif
 
 	if (!fsm_getstart(fsm, &state)) {
 		errno = EINVAL;