refactor(rust): Split Reduction into it plus ReductionState

[orlp]

This also fixes issues like mean overflowing, and the empty sum returning null rather than 0.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 201 lines in your changes missing coverage. Please review.

Project coverage is 79.86%. Comparing base (915f164) to head (2a6409f).
Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/reduce/nan_min_max.rs	0.00%	74 Missing ⚠️
crates/polars-expr/src/reduce/min_max.rs	0.00%	58 Missing ⚠️
crates/polars-stream/src/nodes/reduce.rs	0.00%	23 Missing ⚠️
crates/polars-expr/src/reduce/mean.rs	0.00%	18 Missing ⚠️
crates/polars-expr/src/reduce/sum.rs	0.00%	15 Missing ⚠️
crates/polars-expr/src/reduce/len.rs	0.00%	6 Missing ⚠️
crates/polars-expr/src/reduce/convert.rs	0.00%	4 Missing ⚠️
crates/polars-core/src/scalar/mod.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18460      +/-   ##
==========================================
+ Coverage   79.84%   79.86%   +0.01%     
==========================================
  Files        1497     1498       +1     
  Lines      201828   201805      -23     
  Branches     2867     2867              
==========================================
+ Hits       161141   161163      +22     
+ Misses      40141    40096      -45     
  Partials      546      546              

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