feat: optimize signature readers for most common messages

[bdraco]

• Add specific paths for the 2 most common message types
• Drop all the cast complexity as we can do just as well with cython bytearray
• Works out to ~25% speed up testing on my M1 mac and ~29% speed up on the odroid n2+
• closes Only two high volume body in the whole stack #106

[codecov]

Codecov Report

Base: 81.52% // Head: 81.19% // Decreases project coverage by -0.33% ⚠️

Coverage data is based on head (7e3408c) compared to base (a4fdda2).
Patch coverage: 89.36% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   81.52%   81.19%   -0.34%     
==========================================
  Files          26       26              
  Lines        2994     2988       -6     
  Branches      578      577       -1     
==========================================
- Hits         2441     2426      -15     
- Misses        340      346       +6     
- Partials      213      216       +3     

Impacted Files	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/_private/unmarshaller.py	94.86% <91.30%> (-1.67%)	⬇️
src/dbus_fast/glib/message_bus.py	81.14% <0.00%> (-2.05%)	⬇️

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