Implement permutation view for external sort.

[lums658]

This pull request implements the permutation_view class. A permutation_view provides a view over an underlying range that is permuted by indirecting with a given index range. It is required that the permutation range is the same size as the data range and that it contain the values 0, 1, ..., N-1, in arbitrary order, where N is the size of the data range.

Example:

  std::vector<int> v{1, 2, 3, 4, 5};
  std::vector<size_t> p{4, 3, 2, 1, 0};
  permutation_view view(v, p);

  CHECK(std::ranges::equal(view, std::vector<int>{5, 4, 3, 2, 1}));

The implementation uses iterator_facade to realize a C++20 standards conformant iterator and so that permutation_view satisfies the appropriate C++20 ranges concepts.

The permutation_view class has been tested with var_length_view. Various range and iterator concepts are also tested.

(This PR also cleans up the documentation and the structure of the var_length_view class.)

[sc-43635]

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TYPE: IMPROVEMENT
DESC: Implement permutation view for external sort.