Count self-loops and multi-edges

[seunghwak]

Close #537

Count the number of self-loops & multi-edges.

• API update
• Implement count self-loops
• Implement count multi-edges
• Add C++ test suites.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (branch-21.12@7bffab8). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff               @@
##             branch-21.12    #1939   +/-   ##
===============================================
  Coverage                ?   70.04%           
===============================================
  Files                   ?      143           
  Lines                   ?     8904           
  Branches                ?        0           
===============================================
  Hits                    ?     6237           
  Misses                  ?     2667           
  Partials                ?        0           

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[ChuckHastings]

@gpucibot merge