Use named fields for Wasmi load instructions

[Robbepop]

This will help simplify the integration of #1152.

[codecov]

Codecov Report

Attention: Patch coverage is 40.29126% with 123 lines in your changes missing coverage. Please review.

Project coverage is 79.01%. Comparing base (bb031ed) to head (75a221c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/engine/executor/instrs.rs	50.00%	56 Missing ⚠️
...ates/wasmi/src/engine/translator/visit_register.rs	0.00%	42 Missing ⚠️
...rates/wasmi/src/engine/translator/relink_result.rs	42.85%	24 Missing ⚠️
crates/wasmi/src/engine/executor/instrs/load.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1163      +/-   ##
==========================================
- Coverage   79.17%   79.01%   -0.16%     
==========================================
  Files         296      296              
  Lines       25400    25438      +38     
==========================================
- Hits        20111    20101      -10     
- Misses       5289     5337      +48     

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[Robbepop]

The Miri (spec) CI job fails because it timed out. This is a known issue and we need to either change what the job tests via Miri or increase the timeout limit to fix the flakiness.