Expose `Hierarchy` struct?

https://docs.rs/zarrs/latest/zarrs/hierarchy/struct.Hierarchy.html

Read the linked zarrs Hierarchy documentation first, then inspect zarrista's Python-to-Rust binding entry points to find how comparable structs are exposed. Done means Hierarchy is available through the Python API with behavior matching the documented Rust type; the issue names no specific test or file, so locate the relevant binding tests before changing anything.

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