Move Yoga layout to a separate thread

[vincentriemer]

The major blocker to accomplishing this is that the measure function used primarily for measuring text is synchronous. First idea to try out is to hack together a way to sync-ify the call using while loops and setTimeout.

Another unknown concern is how making the layout async will affect the stability of rendering. It is likely some sort of "locking" functionality will need to be added. Layout performance itself will likely not be improved through the change but considering the the "Native" driven animations are on the main thread they currently can be stalled/blocked by an expensive layout calculation.