Fix mutable nodeList reference during traversal

[veewee]

Q	A
Type	bug
BC Break	no
Fixed issues	#49

Summary

Closes #49

childNodes is a mutable nodeList of which the count gets altered when one of its nodes gets removed whilst iterating.
When one of the linked nodes in the nodeList is removed, the iterator stops.
This breaks the foreach:

https://3v4l.org/FksZo

<?php

$doc = new DOMDocument();
$doc->preserveWhiteSpace = false;
$doc->loadXML(
    <<<EOXML
    <x>
        <item1 />
        <item2 />
        <item3 />
        <item4 />
    </x>
EOXML
);

foreach ($doc->documentElement->childNodes as $i => $node) {
    echo $node->localName . PHP_EOL;
    if ($i === 2) {
        $node->remove();
    }
}

Output:

item1
item2
item3

To make sure we keep track of all original nodes, this PR uses our handy built-in immutable NodeList instead inside the traverser whilst going over the nodes.
Making it possible to make visitors like this:

class RemoveEmptyElementsVisitor extends AbstractVisitor {
    public function onNodeLeave(\DOMNode $node): Action
    {
        if (!is_element($node)) {
            return new Action\Noop();
        }

        if (trim($node->textContent) !== '') {
            return new Action\Noop();
        }

        return new Action\RemoveNode();
    }
}

Those can strip out all empty tags:
From

<movies>
    <movie>
        <name>Terminator</name>
        <genre>
            <action />
            <romance />
        </genre>
        <prices>
            <oscar amount="4" />
        </prices>
    </movie>
</movies>

To

<movies>
    <movie>
        <name>Terminator</name>
    </movie>
</movies>