Refactor Instr and move definition to translator::utils submodule

[Robbepop]

Simplifies #1512 by making more code sharable.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.18%. Comparing base (352d564) to head (d7a228a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/engine/translator/utils.rs	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1541      +/-   ##
==========================================
- Coverage   72.21%   72.18%   -0.03%     
==========================================
  Files         179      179              
  Lines       16406    16408       +2     
==========================================
- Hits        11847    11844       -3     
- Misses       4559     4564       +5     

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