Implement component autosizing

[adam-arold]

With the new option for container builders to add their future children it is now possible to auto-size containers for their children.

[lesbiangunshow]

this also opens the door to autosizing during runtime, right? if we have access to the view hierarchy we could trigger rebuilding when autosized components' children's sizes change, or children are added or removed to ensure constraints continue to be matched. For example, it would be trivial to have a box wrap some row elements, and have it change size to match the amount of elements within.