…th valid_node checks

==> SiblingGraph::new's "get_optype" will implicitly check the root is in the base view.

This does mean some unnecessary index checking for &Hugr's. We could continue to
reimplement those methods for AsRef<Hugr> if we want to avoid this inefficiency.
Alternatively we could make UnmanagedDenseMap return Options rather than unwrapping.

Some other ugly alternative/hacks for returning default values - DEFAULT_{NODE,OP}TYPE

…e_hugr in c'tor

…aseless

…aseless

…EMPTY_GRAPH

…iling without??

…h and collect vec into iter