Add rector, raise PHP to ^8.0, minor refactoring

[xepozz]

Added rector/rector to CI and composer.json. Mass creation refers to yiisoft/yii-dev-tool#232

[codecov]

Codecov Report

Base: 98.94% // Head: 98.91% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (cdbb025) compared to base (6b8c8b2).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head cdbb025 differs from pull request most recent head 4463b95. Consider uploading reports for the commit 4463b95 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #41      +/-   ##
============================================
- Coverage     98.94%   98.91%   -0.04%     
+ Complexity      214      210       -4     
============================================
  Files            14       14              
  Lines           475      461      -14     
============================================
- Hits            470      456      -14     
  Misses            5        5              

Impacted Files	Coverage Δ
src/ArrayDefinition.php	100.00% <100.00%> (ø)
src/CallableDefinition.php	100.00% <100.00%> (ø)
src/DefinitionStorage.php	100.00% <100.00%> (ø)
src/DynamicReference.php	100.00% <100.00%> (ø)
src/Helpers/DefinitionExtractor.php	100.00% <100.00%> (ø)
src/Helpers/DefinitionResolver.php	100.00% <100.00%> (ø)
src/Helpers/DefinitionValidator.php	100.00% <100.00%> (ø)
src/Helpers/Normalizer.php	100.00% <100.00%> (ø)
src/ParameterDefinition.php	95.09% <100.00%> (-0.32%)	⬇️
src/Reference.php	100.00% <100.00%> (ø)
... and 1 more

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