style: reduce the number of files in root directory

[samber]

because you need to scroll too many times before reading the readme

group _example_test.go files into a single file for the root package

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.24%. Comparing base (7dfaa3c) to head (d77a1cf).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #719      +/-   ##
==========================================
- Coverage   94.53%   94.24%   -0.29%     
==========================================
  Files          18       18              
  Lines        3437     2780     -657     
==========================================
- Hits         3249     2620     -629     
+ Misses        174      146      -28     
  Partials       14       14              

Flag	Coverage Δ
unittests	94.24% <ø> (-0.29%)	⬇️

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