Mark classes as final

[AJenbo]

Type: refactoring
Breaking change: yes

A good article on the subject: https://ocramius.github.io/blog/when-to-declare-classes-final/

Marks classes we do not extend as final. This makes it easier to reason about both for humans and tools. And it lets us know that they are not changed by users allowing us more flexibility when refactoring, since the internals cannot have been manipulated by an extending class. This also slightly lowers coupling as it promotes using interfaces and abstracts when making hints (10 couplings got removed as part of this PR).

All classes marked as final in this PR either cannot be replaced, should not be extended, or is never used as types.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.15%. Comparing base (39a9529) to head (9d691d4).

Additional details and impacted files

@@            Coverage Diff            @@
##                3.x    #1152   +/-   ##
=========================================
  Coverage     92.15%   92.15%           
  Complexity     1278     1278           
=========================================
  Files           107      107           
  Lines          3378     3378           
=========================================
  Hits           3113     3113           
  Misses          265      265           

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