Refactor ThinPtr + executor's instance access APIs

[Robbepop]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.34%. Comparing base (d737d7f) to head (1b16ee4).

Files with missing lines	Patch %	Lines
crates/wasmi/src/instance/entity.rs	69.23%	4 Missing ⚠️
...tes/wasmi/src/engine/executor/handler/exec/simd.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2020      +/-   ##
==========================================
+ Coverage   76.31%   76.34%   +0.03%     
==========================================
  Files         166      166              
  Lines       15598    15598              
==========================================
+ Hits        11904    11909       +5     
+ Misses       3694     3689       -5     

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