refactor(rust): Add AExpr input node iterators

[nameexhaustion]

Adds

• AExpr::nodes_iter(_mut)
• AExpr::inputs_iter(_mut) - like nodes_iter but excludes the evaluation exprs of list/eval exprs

Before / after

• ae.children_rev(stack) -> stack.extend(ae.nodes_iter_name_last())
• ae.inputs_rev(stack) -> stack.extend(ae.inputs_iter_name_last())

Other notes

• At the moment inputs_rev() is inconsistent in that it excludes the eval exprs of ListEval but includes the eval exprs of StructEval. In this PR inputs_iter will exclude eval exprs of both variants.

[github-actions]

The uncompressed lib size after this PR is 60.6090 MB.

[github-actions]

The uncompressed lib size after this PR is 60.6530 MB.

[github-actions]

The uncompressed lib size after this PR is 60.4936 MB.

[github-actions]

The uncompressed lib size after this PR is 60.6420 MB.

[github-actions]

The uncompressed lib size after this PR is 60.6596 MB.

[github-actions]

The uncompressed lib size after this PR is 60.6574 MB.

[github-actions]

The uncompressed lib size after this PR is 60.6579 MB.

[codecov]

Codecov Report

❌ Patch coverage is 81.68168% with 61 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.81%. Comparing base (62f38ef) to head (5236a2d).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-plan/src/plans/aexpr/traverse.rs	80.45%	51 Missing ⚠️
crates/polars-plan/src/plans/ir/inputs.rs	50.00%	8 Missing ⚠️
.../polars-plan/src/plans/aexpr/properties/general.rs	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27437      +/-   ##
==========================================
+ Coverage   80.54%   80.81%   +0.27%     
==========================================
  Files        1842     1844       +2     
  Lines      254718   256015    +1297     
  Branches     3181     3177       -4     
==========================================
+ Hits       205173   206910    +1737     
+ Misses      48722    48283     -439     
+ Partials      823      822       -1     

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[github-actions]

The uncompressed lib size after this PR is 61.3423 MB.

[orlp]

I talked about this with Kuba who introduced something similar already, and I'm really not a fan of these complex iterator types with Single/Double/Slice, etc that dispatch... I actually feel the API where you push to an input vector much more elegant. Iterator wrappers could be added for that.

[nameexhaustion]

I actually feel the API where you push to an input vector much more elegant. Iterator wrappers could be added for that.

The main motivation for this was that I ran into a drawback with that where you aren't able to get (a mutable reference to) the node at a particular index without causing all of the nodes to be extended into a vec. With the iterators this is possible via Iterator::nth(idx).

I also find the iterators end up making some code at the callsite cleaner (e.g. in the PR I'd removed 2 struct ExtendWrap(..); impl Extend<Node> for ExtendWrap wrappers. Also is quite handy being able to do ExactSizeIterator::len() to get the number of nodes.