tree/node: manipulate handle of child node via link being inserted
tree/node: manipulate handle of child node via link being inserted

Prev commit adds an API to manage handle of node via link to it -- use
it and don't call `link::data_node()`, because this might be
inefficient.

uentity/blue-sky-re,uentity/blue-sky-re,uentity/blue-sky-re