refactor(rust): Move expression expansion to conversion module

[ritchie46]

No description provided.

[codspeed-hq]

CodSpeed Performance Report

Merging #16331 will improve performances by 15.12%

_{Comparing expand (d6adfd6) with main (f5c32f2)}

Summary

⚡ 1 improvements
✅ 36 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	expand	Change
⚡	test_groupby_h2oai_q4	9.8 ms	8.5 ms	+15.12%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.35%. Comparing base (f5c32f2) to head (d6adfd6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16331   +/-   ##
=======================================
  Coverage   81.35%   81.35%           
=======================================
  Files        1403     1403           
  Lines      183707   183707           
  Branches     2954     2954           
=======================================
+ Hits       149460   149461    +1     
+ Misses      33736    33735    -1     
  Partials      511      511           

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