Render correct HTML5

⚠️ Breaking: The rendering is changed to correctly render HTML5. This includes not rendering an end tag for void elements (see https://dev.w3.org/html5/spec-LC/syntax.html#void-elements), and always rendering an end tag for all other elements. This means that <script> and <textarea> elements now render and work as expected, and that elements like <hr> and <br> render without a slash in them.

Also breaking: The Placer interface has been removed. Implement Type() NodeType instead if you need to create e.g. attribute helpers.