Fix minor findings

[Robbepop]

• correctly spell [un]signed.
• fix comment for last_index - we may also want to find a better name.
• remove unused method adjust_for_call.
• fix comment stating store instead of load.
• re-instate panic for CompiledFuncEntity::new when there are more than i32::MAX instructions.
• BranchOffset API: remove invalid comment about panics.
• CodeMap::get_compiled: conditionally panic if debug_assertions are enabled.

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.35%. Comparing base (adedd6b) to head (a68321d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/src/engine/code_map.rs	33.33%	2 Missing ⚠️
crates/wasmi/src/engine/utils.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1207   +/-   ##
=======================================
  Coverage   81.35%   81.35%           
=======================================
  Files         302      303    +1     
  Lines       24953    24951    -2     
=======================================
  Hits        20300    20300           
+ Misses       4653     4651    -2     

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