Support creating static ArcStr's at runtime (leaking them)

[thomcc]

I wrote this a while ago, then didn't need it, but it may be useful to others.

The main drawback is that it adds a branch to refcount manipulation, but that should be a wash (the function is already doing atomic RMWs, which are likely more costly than a the branching most of the time).

On the positive side, it makes it so we no longer need to abort on refcount overflow -- we can simply transition the string to static (leak it).

I've also updated some deps and may do a bit of other cleanup while I'm here.

EDIT: this is now mostly cleanup.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 52.94118% with 48 lines in your changes are missing coverage. Please review.

Project coverage is 83.22%. Comparing base (36dcce9) to head (b05a84b).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/arc_str.rs	52.94%	48 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   92.48%   83.22%   -9.27%     
==========================================
  Files           4        4              
  Lines         386      447      +61     
==========================================
+ Hits          357      372      +15     
- Misses         29       75      +46     

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