fix: Hierarchy descendants return root siblings

[aborgna-q]

Closes #177

[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.27%. Comparing base (fbd65b7) to head (d1b03e1).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hierarchy.rs	84.21%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #178   +/-   ##
=======================================
  Coverage   83.27%   83.27%           
=======================================
  Files          24       24           
  Lines        6104     6117   +13     
  Branches     6104     6117   +13     
=======================================
+ Hits         5083     5094   +11     
- Misses        945      947    +2     
  Partials       76       76           

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[lmondada]

I suggest you turn child_queue and root into a sum type, but if you think it's more hassle than it's worth, then this is fine by me.

[lmondada]

I let you be the judge of whether this is worth it, but in principle I'd much prefer something like:

enum NextStates {
    Root(NodeIndex),
    Children(VecDeque<NodeIndex>),
}

pub struct Descendants<'a> {
    /// The hierarchy this iterator is iterating over.
    layout: &'a Hierarchy,
    /// A queue of regions to visit.
    ///
    /// For each region, we point to a child node that has not been visited yet.
    /// When a region is visited, we move to the next child node and queue its children region at the end of the queue.
    next_states: NextStates,
}

It would make it clearer what the purpose of each field is, and will also make the Default implementation more sane.

[aborgna-q]

Sure, makes sense.