refactor(rust): Simplify decimal formatting and remove itoap dep

[orlp]

Possibly a bit slower but I doubt it's much different or it forming a bottleneck. Can always optimize it more later.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.72%. Comparing base (b64b764) to head (1499965).
Report is 29 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-compute/src/cast/decimal_to.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20880      +/-   ##
==========================================
- Coverage   79.73%   79.72%   -0.01%     
==========================================
  Files        1566     1566              
  Lines      222574   222559      -15     
  Branches     2562     2562              
==========================================
- Hits       177460   177445      -15     
  Misses      44525    44525              
  Partials      589      589              

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