feat: speed up unmarshalling message body

[bdraco]

This is ~3.4% speed up

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.67% 🎉

Comparison is base (dd74a84) 82.68% compared to head (c8e9b96) 83.35%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
+ Coverage   82.68%   83.35%   +0.67%     
==========================================
  Files          28       28              
  Lines        3275     3281       +6     
  Branches      678      682       +4     
==========================================
+ Hits         2708     2735      +27     
+ Misses        341      333       -8     
+ Partials      226      213      -13     

Files Changed	Coverage Δ
src/dbus_fast/_private/unmarshaller.py	94.18% <100.00%> (+4.56%)	⬆️

... and 1 file with indirect coverage changes

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