LowPriorityImplicits

Implicit conversions from X to Inserter are primarily needed for onMountInsert, but they are relatively expensive compared to simpler alternatives when a mere Modifier would suffice. And so, the conversions below are de-prioritized.

#Note: Prior to v17, these conversions involved dynamic inserters with inefficient implementations like children <-- Val(nodes.toList). Now, they use static inserters, and don't have such a significant inefficiency. #TODO Simplify this! See the other #TODO comment above about moving stuff out of LowPriorityImplicits.