Mb/early traversal termination

[MBauerDC]

Allow early termination of tree-traversals in visitors via throwing EarlyTraversalTerminationException in visitor-function and catching it in the caller to TreeNodeVisitor->visit[Order]Order.

Can be used like:

$someIdToTerminateOn = 'foo';
$exists = false;
$earlyTerminatingFunc = static function(TreeNode $n) use ($someIdToTerminateOn, $exists) {
    if ($someIdToTerminateOn === $n->getId()) {
        $exists = true;
        throw new EarlyTraversalTerminationException();
    }
} 
$visitor = new GenericTreeNodeVisitor($earlyTerminatingFunc);
try {
    $visitor->visitPreOrder($tree);
} catch (EarlyTraversalTerminationException) {}

For large trees and/or visitor-functions which take a lot of time per visited node, this will significantly increase performance.

NOTE: Long term, a better solution is to refactor the methods to return null|EarlyTraversalTermination (refactor exception to empty response class) - but that requires chaning the signature, which should be for another task.