refactor(rust): Remove last instances of itoa

[orlp]

Another dependency down.

[codecov]

Codecov Report

Attention: Patch coverage is 97.91667% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.73%. Comparing base (7ccb3ae) to head (58a2fd5).
Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-arrow/src/compute/decimal.rs	97.77%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20881   +/-   ##
=======================================
  Coverage   79.73%   79.73%           
=======================================
  Files        1566     1566           
  Lines      222591   222604   +13     
  Branches     2572     2572           
=======================================
+ Hits       177473   177485   +12     
- Misses      44526    44527    +1     
  Partials      592      592           

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