feat: Include private HUGR functions in the local symbol table

[cgh-qtnm]

This PR changes the LLVM linkage type for functions with HUGR Visibility::Private from Linkage::Private to Linkage::Internal. The only difference between the two types is that Internal functions show up in the local symbol table of the resulting binary, while Private functions do not.

This change makes it easier to debug and profile compiled HUGR programs, by enabling the inclusion of HUGR functions in call stacks generated by a debugger or profiler. I see several other linkage-related PRs around but none of them seem focused on this exact issue - let me know if this is incorrect.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.64%. Comparing base (15b27cd) to head (f71be1e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2831   +/-   ##
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  Coverage   83.64%   83.64%           
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  Files         261      261           
  Lines       52837    52837           
  Branches    47372    47372           
=======================================
  Hits        44195    44195           
  Misses       6253     6253           
  Partials     2389     2389           

Flag	Coverage Δ
python	87.97% <ø> (ø)
rust	83.14% <100.00%> (ø)

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