Reorganize the tags for tree serialization.
Reorganize the tags for tree serialization.

The order now matches the order of definitions of `Tree` case
classes.

nicolasstucki/scala-js,scala-js/scala-js,gzm0/scala-js,gzm0/scala-js,scala-js/scala-js,SebsLittleHelpers/scala-js,sjrd/scala-js,sjrd/scala-js,scala-js/scala-js,sjrd/scala-js,SebsLittleHelpers/scala-js,nicolasstucki/scala-js