refactor!: Change assertion traits to define spec-like return types as associated types

[haraldmaida]

Currently some assertion traits define "mapping" assertion methods. These are methods that do not return Self, but a Spec holding a different subject as a result of mapping the original subject.

This API shall be more flexible in that the implementor can define the concrete return type. Therefore assertion traits where the methods have a return type other than Self shall use associated types as the methods return types. This allows that the implementor can define the concrete return types.

This refactoring is also a necessary preparation for upcoming more advanced features of asserting.

BREAKING: as the definition of some assertion traits are changed, any custom implementations of the refactored assertion traits must be adapted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.85%. Comparing base (62eb4c9) to head (51a9394).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
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- Coverage   92.86%   92.85%   -0.01%     
==========================================
  Files          41       41              
  Lines        5759     5756       -3     
  Branches     5759     5756       -3     
==========================================
- Hits         5348     5345       -3     
  Misses        345      345              
  Partials       66       66              

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