matrices: simplify gen_pauli_x construction

[atomgardner]

Eliminate the reassignments in the gen_pauli_x construction by rolling along an axis.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.1%. Comparing base (d1049fe) to head (0d0ab74). Report is 21 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #545     +/-   ##
========================================
- Coverage    98.1%   98.1%   -0.1%     
========================================
  Files         162     162             
  Lines        3112    3108      -4     
  Branches      760     760             
========================================
- Hits         3054    3050      -4     
  Misses         37      37             
  Partials       21      21             

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