feat: improve performance of unmarshalling headers

[bdraco]

This is only ~5% but its a tiny change

[codecov]

Codecov Report

Base: 81.56% // Head: 81.81% // Increases project coverage by +0.25% 🎉

Coverage data is based on head (f27f0a4) compared to base (f554345).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   81.56%   81.81%   +0.25%     
==========================================
  Files          25       25              
  Lines        2935     2937       +2     
  Branches      566      566              
==========================================
+ Hits         2394     2403       +9     
+ Misses        333      328       -5     
+ Partials      208      206       -2     

Impacted Files	Coverage Δ
src/dbus_fast/_private/unmarshaller.py	96.47% <100.00%> (+0.03%)	⬆️
src/dbus_fast/message_bus.py	73.19% <0.00%> (+0.76%)	⬆️
src/dbus_fast/proxy_object.py	78.52% <0.00%> (+2.01%)	⬆️

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