Add CodeMap::alloc_funcs API

[Robbepop]

The new CodeMap::alloc_funcs replaces the CodeMap::alloc_func API.

The advantage is that fewer CodeMap locks are required to allocate multiple EngineFunc at the same time which is the only way CodeMap::alloc_func was used.

Also this PR introduces the new EngineFuncSpan and EngineFuncSpanIter abstractions which significantly improve space efficiency of storing newly allocated contiguous EngineFunc indices returned by CodeMap::alloc_funcs.

This allowed us to optimize the way ModuleHeader and ModuleBuilder store and manage allocated EngineFuncs.

Benchmarks conducted locally confirm that this is a huge performance improvement, especially for lazy Wasm module compilation:

[codecov]

Codecov Report

Attention: Patch coverage is 83.13253% with 14 lines in your changes missing coverage. Please review.

Project coverage is 80.56%. Comparing base (b42fd3b) to head (146fa9e).

Files	Patch %	Lines
crates/wasmi/src/engine/code_map.rs	77.77%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1125      +/-   ##
==========================================
+ Coverage   80.54%   80.56%   +0.02%     
==========================================
  Files         273      273              
  Lines       25087    25080       -7     
==========================================
  Hits        20206    20206              
+ Misses       4881     4874       -7     

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