Change OpCostMetrics.memory to be a nonnegative_int

[victorli2002]

Description of changes:

• Change OpCostMetrics.memory to be a nonnegative_int
• Add nonnegative_int + nonnegative_int addition operator

Related Issues:

Linked Issues:

• Issue #

Issues closed by this PR:

• Closes Change OpCostMetrics.memory to be a nonnegative_int #1584

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This change is 

[lockshaw]

Reviewed 7 of 7 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @wmdi)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.47%. Comparing base (34f3848) to head (efc73f5).
Report is 11 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1586   +/-   ##
=======================================
  Coverage   60.47%   60.47%           
=======================================
  Files         606      606           
  Lines       14723    14725    +2     
=======================================
+ Hits         8903     8905    +2     
  Misses       5820     5820           

Flag	Coverage Δ
unittests	60.47% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...utils/src/utils/nonnegative_int/nonnegative_int.cc	96.66% <100.00%> (+0.11%)	⬆️