Use named fields for some Instruction variants

[Robbepop]

Simplifies #1152 integration later on.

[codecov]

Codecov Report

Attention: Patch coverage is 43.17549% with 204 lines in your changes missing coverage. Please review.

Project coverage is 79.17%. Comparing base (3f9bb82) to head (e6f6eee).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/engine/executor/instrs.rs	38.46%	80 Missing ⚠️
...ates/wasmi/src/engine/translator/visit_register.rs	3.03%	64 Missing ⚠️
crates/wasmi/src/engine/executor/instrs/branch.rs	4.76%	40 Missing ⚠️
...rates/wasmi/src/engine/translator/instr_encoder.rs	72.46%	19 Missing ⚠️
...rates/wasmi/src/engine/translator/relink_result.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1162      +/-   ##
==========================================
- Coverage   79.29%   79.17%   -0.12%     
==========================================
  Files         296      296              
  Lines       25357    25400      +43     
==========================================
+ Hits        20106    20111       +5     
- Misses       5251     5289      +38     

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