Add client to count may-alias variable pairs

[jjppp]

This PR proposes a client for counting the number of variable pairs that are may-aliases.

Formally, a pair of variables u and v are considered may-aliases if the points-to-set of u and v (i.e., pts(u) and pts(v)) are not disjoint.
The number of may-alias variables is often used as a metric to evaluate the precision of a pointer analysis algorithm.

In this PR, a pointer analysis client MayAliasPair is implemented to compute the number of may-alias variable pairs. The proposed implementation is parallel and should be faster than the naive implementation used in the unit test MayAliasPairTest, who simply iterates all variable pairs and test if their points-to-sets are disjoint.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.80%. Comparing base (e3eadc9) to head (4fb151e).

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #149      +/-   ##
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+ Coverage     71.90%   72.80%   +0.89%     
- Complexity     4362     4430      +68     
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  Files           479      480       +1     
  Lines         15869    15917      +48     
  Branches       2179     2181       +2     
============================================
+ Hits          11411    11588     +177     
+ Misses         3575     3480      -95     
+ Partials        883      849      -34     

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[zhangt2333]

Minor Suggestion(s):

• Consider renaming numberOfAliasPairs to aliasPairs for brevity. Similar situations apply to other cases.