Add unique and improve backends

[polazarus]

• add unique smart pointer
• add more tests
• add loom tests

[codecov]

Codecov Report

Attention: Patch coverage is 99.56710% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.78%. Comparing base (7432c7f) to head (0919482).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/smart.rs	99.35%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   99.82%   99.78%   -0.04%     
==========================================
  Files          24       24              
  Lines        3337     3325      -12     
  Branches     3337     3325      -12     
==========================================
- Hits         3331     3318      -13     
- Misses          1        2       +1     
  Partials        5        5              

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