feat: speed up decoding headers by avoiding unicode checks

[bdraco]

~4.5% speed up decoding all messages since its the header

[codecov]

Codecov Report

Base: 81.92% // Head: 81.78% // Decreases project coverage by -0.13% ⚠️

Coverage data is based on head (21ee6a3) compared to base (c8a9452).
Patch coverage: 87.50% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #125      +/-   ##
==========================================
- Coverage   81.92%   81.78%   -0.14%     
==========================================
  Files          27       27              
  Lines        3087     3091       +4     
  Branches      597      597              
==========================================
- Hits         2529     2528       -1     
- Misses        340      343       +3     
- Partials      218      220       +2     

Impacted Files	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/_private/unmarshaller.py	95.29% <100.00%> (+0.05%)	⬆️
src/dbus_fast/glib/message_bus.py	81.14% <0.00%> (-2.05%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.