Feature/token wrapping

[johnabass]

This PR implements Token wrapping, a feature very similar to the errors package.

It's often true that authentication needs to augment a Token. This need can arise because of reading from an external database, needing to add attributes for the application layer to process, etc. You can now supply an Unwrap method in custom tokens to communicate nesting. The TokenAs function allows client code to extract custom types from tokens in a way much like errors.As does for errors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3e40273) to head (69196cf).
Report is 4 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           21        21           
  Lines          733       789   +56     
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+ Hits           733       789   +56     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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