fix: Properly implement struct

[ritchie46]

This will fix outer nullability issues and lot's of bugs relating to chunking. The struct type will now be first class citizen.

This touches a lot of code, so I am going to give this one preference in merging as conflicts are a nightmare in this.

[codecov]

Codecov Report

Attention: Patch coverage is 80.10152% with 196 lines in your changes missing coverage. Please review.

Project coverage is 80.39%. Comparing base (a5724d6) to head (e27d67e).
Report is 5 commits behind head on main.

Files	Patch %	Lines
...tes/polars-arrow/src/array/static_array_collect.rs	0.00%	30 Missing ⚠️
...polars-core/src/series/implementations/struct__.rs	84.94%	28 Missing ⚠️
crates/polars-ops/src/series/ops/is_in.rs	62.29%	23 Missing ⚠️
...rates/polars-core/src/chunked_array/struct_/mod.rs	94.69%	14 Missing ⚠️
...es/polars-core/src/chunked_array/comparison/mod.rs	79.36%	13 Missing ⚠️
crates/polars-arrow/src/array/static_array.rs	0.00%	12 Missing ⚠️
...ates/polars-core/src/chunked_array/ops/downcast.rs	0.00%	8 Missing ⚠️
crates/polars-core/src/chunked_array/ops/mod.rs	69.23%	8 Missing ⚠️
crates/polars-ops/src/frame/join/merge_sorted.rs	42.85%	8 Missing ⚠️
py-polars/src/map/series.rs	63.63%	8 Missing ⚠️
... and 13 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17522      +/-   ##
==========================================
- Coverage   80.47%   80.39%   -0.09%     
==========================================
  Files        1482     1483       +1     
  Lines      195119   195160      +41     
  Branches     2778     2778              
==========================================
- Hits       157026   156900     -126     
- Misses      37582    37749     +167     
  Partials      511      511              

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