Remove some deprecated baggage + more readonly/final

• Make many classes readonly and/or final, where it is clear that it is the right choice
• Refactor some of the internal code to use match, str_contains and other more modern ways to write code
• Add missing test to get back to 100% code coverage
• Generate classes with readonly and final where possible
• Remove PopulatePropertiesWithIterableTrait, as constructor property promotion and named arguments should be used instead
• Wrong type coercions now throw exceptions instead of deprecation notices