Node visitation is now properly nested. The call sequence will
now be
$visitor1->enterNode($n);
$visitor2->enterNode($n);
$visitor2->leaveNode($n);
$visitor1->leaveNode($n);
rather than
$visitor1->enterNode($n);
$visitor2->enterNode($n);
$visitor1->leaveNode($n);
$visitor2->leaveNode($n);
Fixes#899.