Extract the type layout graph without using the unstable+nightly specialization feature

[juntyr]

• clean up the code and add type aliases to reduce the &'static [&'static TypeLayoutInfo<'static>] clutter
• add documentation
• decide if we want to keep the hlist![...] naming
• decide what to expose in the graph module
• check if we can replace the remaining pub macro with macro_rules

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 163 lines in your changes missing coverage. Please review.

Project coverage is 15.36%. Comparing base (d22f76d) to head (e759138).

Files with missing lines	Patch %	Lines
src/graph.rs	0.00%	161 Missing ⚠️
src/lib.rs	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
- Coverage   21.81%   15.36%   -6.45%     
==========================================
  Files           3        4       +1     
  Lines         353      501     +148     
==========================================
  Hits           77       77              
- Misses        276      424     +148     

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