Revisit the Region filter

[aborgna-q]

Region and FlatRegion are just NodeFiltered specialisations with methods that filter nodes based on the hierarchy.
This is sometimes inefficient, as it skips possible optimisations we could have:

• FlatRegion::nodes could return a hierarchy::Children iterator directly, without traversing the full graph.
• Similarly,Region::nodes could do a direct bfs traversal without reading the cache each time.
• The RefCell in the Region traversal is a bit iffy. Is it multithreading-safe? Can we replace it with something simpler / with support for parallel access?