Faster pair hash for ACM

[rjoomen]

Before:

After:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.91%. Comparing base (7a971fb) to head (a7fed35).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #992      +/-   ##
==========================================
- Coverage   90.92%   90.91%   -0.01%     
==========================================
  Files         280      280              
  Lines       15877    15876       -1     
==========================================
- Hits        14436    14434       -2     
- Misses       1441     1442       +1     

see 2 files with indirect coverage changes