feat: speed up unmarshalling Variants

[bdraco]

This is a 3.5% speed up, but we can apply the same idea in a few more places

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: -0.46% ⚠️

Comparison is base (f9b61b8) 83.29% compared to head (5a48d02) 82.83%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #254      +/-   ##
==========================================
- Coverage   83.29%   82.83%   -0.46%     
==========================================
  Files          28       28              
  Lines        3269     3275       +6     
  Branches      678      678              
==========================================
- Hits         2723     2713      -10     
- Misses        330      338       +8     
- Partials      216      224       +8     

Files Changed	Coverage Δ
src/dbus_fast/__version__.py	0.00% <0.00%> (ø)
src/dbus_fast/_private/unmarshaller.py	89.62% <35.29%> (-5.36%)	⬇️

... and 2 files with indirect coverage changes

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[bdraco]

production testing looks good