Added PermutationUnaryOperator and PermutationBinaryOperator interfaces and related

[cicirello]

Summary

This PR contains the following:

Added

• PermutationUnaryOperator and PermutationBinaryOperator functional interfaces for the purpose
  of specifying custom operations on Permutation objects.
• Permutation.apply methods, one for each of the two new PermutationUnaryOperator
  and PermutationBinaryOperator interfaces, for applying such custom Permutation operators.

Changed

• Various improvements to the documentation.

Deprecated

• The Permutation.Mechanic nested class, which will be removed in the next
  major release 4.0.0. The new Permutation.apply methods should be used instead.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvements to existing code, such as refactoring or optimizations (non-breaking)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[github-actions]

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