Add xmpz.bit_count()

[skirpichev]

Closes #493

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.50%. Comparing base (224745b) to head (6c414dd).
Report is 32 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #494      +/-   ##
==========================================
- Coverage   85.52%   85.50%   -0.03%     
==========================================
  Files          50       50              
  Lines       11656    11646      -10     
  Branches     2202     2201       -1     
==========================================
- Hits         9969     9958      -11     
- Misses       1687     1688       +1     

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[casevh]

Thanks.