Pre-allocate res_units vector

[d-e-s-o]

Commit d0b99be ("Pre-allocate unit_ranges Vec with aranges-based capacity") improved DWARF parsing performance by sizing the unit_ranges vector based on the number of .debug_aranges entries. Resizing the res_units Vec is even worse in terms of performance, because the contained Unit objects are rather large (making copying more expensive).
Use the same size hint to pre-allocate said vector and minimize copy operations in the process.

> main/symbolize_dwarf_no_lines
>      time:   [7.1269 ms 7.1522 ms 7.1901 ms]
>      change: [−30.399% −29.940% −29.484%] (p = 0.00 < 0.02)
>      Performance has improved.
> main/symbolize_dwarf
>      time:   [6.9546 ms 6.9988 ms 7.0619 ms]
>      change: [−34.973% −34.455% −33.805%] (p = 0.00 < 0.02)
>      Performance has improved.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.32%. Comparing base (2ced358) to head (81b9f2d).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1586      +/-   ##
==========================================
+ Coverage   96.21%   96.32%   +0.10%     
==========================================
  Files          56       56              
  Lines       11132    11132              
==========================================
+ Hits        10711    10723      +12     
+ Misses        421      409      -12     

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