internal: Fix a severe bug with the file-system wrappers on OSX. `os.walk` is expected to support modifying its return values `dirnames` and `filenames` so that you can filter out items that need to be iterated upon. I was previously returning a different list in the wrappers, which means filtering was completely broken.

{% extends "section-page.html" %}

{% block header %}
<header class="code">
    <h1>{{ page.title }}</h1>
</header>
{% endblock %}