Some string cleanups

[CrazyboyQCD]

• Simplify JsString::as_str (miri passed, decrease the size of cli executable by ~5kb under release mode (28,680kb => 28,675kb) )
• Replace soft deprecated addr_of* macro with & [const|mut] raw
• Mark some &mut self tagged methods const

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 52.95%. Comparing base (6ddc2b4) to head (518d9ad).
Report is 382 commits behind head on main.

Files with missing lines	Patch %	Lines
core/string/src/lib.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4090      +/-   ##
==========================================
+ Coverage   47.24%   52.95%   +5.71%     
==========================================
  Files         476      488      +12     
  Lines       46892    52465    +5573     
==========================================
+ Hits        22154    27783    +5629     
+ Misses      24738    24682      -56     

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[jedel1043]

Thank you! Looks good!