Defer node reconstruction more when cached children can be used

[mattbierner]

down edit up down

Reconstructs the parent node even though we only need the child list of the node. This info is already in the focus even without a fresh node object. Only when a node is extracted or we move off that node must is be really reconstructed.

[mattbierner]

Although down edit up down seems like a stupid thing to do, the Khepri-compiler actually ends up doing this a lot because of how it expresses visiting child nodes in the composition of high level ops. Expressing the same ops to perform better result in less natural code.