Avoid unnecessary calls of to_string in cow_*

[CrazyboyQCD]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 52.95%. Comparing base (6ddc2b4) to head (7a1108b).
Report is 375 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/string/mod.rs	0.00%	7 Missing ⚠️
core/engine/src/builtins/number/mod.rs	75.00%	1 Missing ⚠️
.../engine/src/builtins/temporal/zoneddatetime/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4166      +/-   ##
==========================================
+ Coverage   47.24%   52.95%   +5.71%     
==========================================
  Files         476      488      +12     
  Lines       46892    52472    +5580     
==========================================
+ Hits        22154    27789    +5635     
+ Misses      24738    24683      -55     

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[jedel1043]

Looks great!

[nekevss]

LGTM