implement xargs -P using rayon and a spin lock

[asakatida]

if this is accepted then I could do #112 in a similar way

[asakatida]

this is meat of the change, the other option here to avoid would be to build the thread pool directly which would avoid needing a lock but I feel the impl Iterator is a cleaner abstraction and is in line with rayon advice

[asakatida]

if more tests are desired then I have a branch ready to push

[codecov]

Codecov Report

❌ Patch coverage is 70.24793% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.32%. Comparing base (1266b7f) to head (e175c3c).

Files with missing lines	Patch %	Lines
src/xargs/mod.rs	70.24%	32 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #653      +/-   ##
==========================================
- Coverage   91.76%   91.32%   -0.44%     
==========================================
  Files          31       31              
  Lines        6203     6283      +80     
  Branches      328      329       +1     
==========================================
+ Hits         5692     5738      +46     
- Misses        390      422      +32     
- Partials      121      123       +2     

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