Use named fields for {Elem,Data}Drop instructions

[Robbepop]

Simplifies integration of #1152.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.65%. Comparing base (40b5f74) to head (505c0ff).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/engine/bytecode/visit_regs.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1182   +/-   ##
=======================================
  Coverage   79.64%   79.65%           
=======================================
  Files         298      298           
  Lines       25292    25295    +3     
=======================================
+ Hits        20145    20149    +4     
+ Misses       5147     5146    -1     

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