Node::update_move and Node::update_swap

[cyrush]

Node::update_move()
Node::update_swap()

Supports combined semantics of update and move/swap.

construct new children in a separate node, and assume them efficiently

use case: iterate over children -- while creating new nodes that we want to become children. assume new children after iteration (avoid invalidation iterator via insertion)

[cyrush]

@agcapps idea related to your use case to discuss in the future.