Improve performance of random_pauli! by 100x

[jlapeyre]

This commit improves the performance of the unbiased branch of random_pauli! by a factor of about 100. Each chunk in the data array is replaced with a random UInt64.

Before:

julia> @time random_pauli(10^9);
  9.596375 seconds (5 allocations: 238.419 MiB, 0.06% gc time)

After:

julia> @btime random_pauli(10^9);
  74.858 ms (5 allocations: 238.42 MiB)

Note:

I included code to unset the noncoding bits in the xz field of PauliOperator. Similar code may already exist somewhere. But I did not find it.

I was unable to find information on details of code formatting.

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[Krastanov]

Thank you, this is great!

For the record, no, we do not have neat masking of the unused bits anywhere else yet. It would be good to be more diligent about that, as currently it can lead to incorrect use in some of the non-public APIs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.73%. Comparing base (6065fab) to head (dd4f232).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
- Coverage   82.82%   82.73%   -0.09%     
==========================================
  Files          70       70              
  Lines        4651     4656       +5     
==========================================
  Hits         3852     3852              
- Misses        799      804       +5     

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