fix: stop generating a QubitSet for each call of qubit_count and qubits

[AbeCoull]

Issue #, if available:

Description of changes: Reduce overhead of creating new qubit sets each time we call qubits or qubit_set

Testing done:

Merge Checklist

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[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (4e9ae75) to head (bcf7945).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #913   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files          132       132           
  Lines         8823      8824    +1     
  Branches      1986      1986           
=========================================
+ Hits          8823      8824    +1     

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