Refactor RawJsString's representation to make JsStrings construction from string literal heap-allocation free

[CrazyboyQCD]

This changes RawJsString's representation, RawJsString in JsString is now a memory variant of RawJsString and StaticJsString which holds a string literal. It has a few benefits we can gain:

• Memory-safe (passes miri, thanks to @jedel1043 )
• JsString could be constant now
• No allocation when JsString is created from string literal by using js_string macro

[codecov]

Codecov Report

Attention: Patch coverage is 79.16667% with 10 lines in your changes missing coverage. Please review.

Project coverage is 51.57%. Comparing base (6ddc2b4) to head (1cbec41).
Report is 221 commits behind head on main.

Files	Patch %	Lines
core/string/src/lib.rs	78.26%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3935      +/-   ##
==========================================
+ Coverage   47.24%   51.57%   +4.33%     
==========================================
  Files         476      468       -8     
  Lines       46892    44983    -1909     
==========================================
+ Hits        22154    23202    +1048     
+ Misses      24738    21781    -2957     

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[nekevss]

This is looking fantastic overall! Thanks for all the effort you put into this!

I still want to play around with it a bit more in depth, but thought I'd leave a couple comments.

[jedel1043]

Implementation wise looks perfect! I just have some suggestions around some safety comments that could be improved.

[CrazyboyQCD]

@jedel1043 , I've improved the comment, if you don't think it's good enough, please continue to point it out.

[nekevss]

Overall, this PR looks fantastic. Great work on this!

I saw the test added, but I still find myself wishing there were more tests. I'm not entirely sure that's not necessary to merge, imo. Maybe it would be a good follow up or first issue for someone? The docs improvements are also really nice as well.

As a separate note, @jedel1043 this may be a good candidate to build out design docs for per #3938.

[jedel1043]

Looks perfect to me!

@nekevss Yeah, I'll open an issue for general improvements around documenting the current design of JsString.