Implement a view into a hugr for SiblingSubgraph

[lmondada]

Now that views::SiblingSubgraph no longer stores a reference to the Hugr, it is impossible to inspect the subgraph of the Hugr (e.g. exploring the subgraph).

For usability it would be helpful to have a method subgraph.as_subhugr(&hugr) or similar to access the underlying sub-Hugr.