• Fetch module and API heavily changed (breaking)
• Added support for request​Animation​Frame, which improves render performance,
  especially for animations
• Styles no longer implicitly add px. Added unit! macro in its place.
• Map can now be used directly in elements, without needing to annotate type and collect
  (ie for child Elements, and Listeners)
• Fixed a bug where empty elements at the top-level were rendering in the wrong order
• Added an empty! macro, which is similar to seed::empty
• Attributes and style now retain order