Remove allocations from HostDefined::get_many_mut

[jedel1043]

Small improvement to that method that avoids creating a Vec for indexing.

I also checked the generated assembly and it seems to skip some panics thanks to the const generic, so that's good.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	95,960	95,960	0
Passed	76,572	76,572	0
Ignored	18,477	18,477	0
Failed	911	911	0
Panics	0	0	0
Conformance	79.80%	79.80%	0.00%

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (37a9046) 47.35% compared to head (306cf79) 47.35%.

Files	Patch %	Lines
core/engine/src/host_defined.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3606   +/-   ##
=======================================
  Coverage   47.35%   47.35%           
=======================================
  Files         475      475           
  Lines       45919    45919           
=======================================
  Hits        21746    21746           
  Misses      24173    24173           

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[HalidOdat]

Nice change! Looks good to me!