fix SharedPtr block destruction

[jxy-s]

SharedPtr was not correctly handling destruction of the block. The allocator would be destructed alongside the compressed pair when the strong reference went to zero. Then it would attempt to free using the allocator it just destructed. This is a problem for stateful allocators as it may have internal state that is rendered invalid after destruction.

The fix is to only destruct the managed object when the strong reference goes to zero. Then, when the weak reference goes to zero, take a copy of the allocator and destruct the remaining parts (including the internal allocator). Then use the copied allocator to carry out the free.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.05%. Comparing base (6a49edb) to head (834c7d3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   91.08%   91.05%   -0.04%     
==========================================
  Files          17       17              
  Lines        1930     1934       +4     
  Branches      235      236       +1     
==========================================
+ Hits         1758     1761       +3     
  Misses         14       14              
- Partials      158      159       +1     

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