Copying Map in `insert_view_link_by_node`

[acl-cqc]

Following #2792 we are copying a HashMap into a BTreeMap to maintain determinism, as the HashMap (NodeLinkingDirectives) is a public type alias. Changing this to BTreeMap would be breaking but would allow avoiding the copy, here:

hugr/hugr-core/src/hugr/linking.rs

Lines 44 to 52 in 370dbc2

	// (This may be slow for large numbers of module-children, but not total size of Hugr)
	let mut nodes = BTreeSet::new();
	if parent.is_some() {
	nodes.extend(other.entry_descendants());
	}
	nodes.extend(children.iter().flat_map(|(&ch, dirv)| match dirv {
	NodeLinkingDirective::Add { .. } => Either::Left(other.descendants(ch)),
	NodeLinkingDirective::UseExisting(_) => Either::Right(std::iter::once(ch)),
	}));