Use named field for Wasmi store instructions

[Robbepop]

Simplifies integration of #1152.

[codecov]

Codecov Report

Attention: Patch coverage is 46.66667% with 72 lines in your changes missing coverage. Please review.

Project coverage is 79.05%. Comparing base (82d7fae) to head (e7bddc2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ates/wasmi/src/engine/translator/visit_register.rs	0.00%	41 Missing ⚠️
crates/wasmi/src/engine/executor/instrs.rs	65.75%	25 Missing ⚠️
crates/wasmi/src/engine/executor/instrs/store.rs	61.53%	5 Missing ⚠️
...rates/wasmi/src/engine/translator/relink_result.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1164      +/-   ##
==========================================
+ Coverage   79.01%   79.05%   +0.03%     
==========================================
  Files         296      296              
  Lines       25438    25418      -20     
==========================================
- Hits        20101    20095       -6     
+ Misses       5337     5323      -14     

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