refact!: move lock_api under barging

[pedromfedricci]

Breaking changes:

• Move root level lock_api module under the barging module. The aliases' inner types have not changed, that is, they are still the barging::Mutex and barging::MutexGuard types. Moving the module under barging reinforces that those aliases refer to a context-free, not FIFO guaranteed MCS lock implementation.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.51%. Comparing base (51ac2c6) to head (1b385e2).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
+ Coverage   99.02%   99.51%   +0.48%     
==========================================
  Files           5        5              
  Lines         205      205              
==========================================
+ Hits          203      204       +1     
+ Misses          2        1       -1     

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