parallels insert_hugr_nodes but we'd want two trait methods, one for Hugr one for View, to handle copying-vs-moving node weights/data.

In fact the reroot callback could be a HashMap<Node, Self::Node>: the method is inserting a forest, and the map contains one entry per root of the inserted forest, so not many entries.

These methods would then be implemented for Hugr by each calling the existing insert_hugr_nodes plus its own bit for copying/moving node weights/data.

insert_hugr and insert_region etc. can then all be trait-default-implemented on top of that (using entry_descendants and a single mapping from inserted entrypoint to parent to preserve current behaviour).