add join statistics

[ben-schwen]

Closes #4677

Not happy that we always call the merge arguments x and i now, even for merge(x,y), but evaluating names of passed arguments seemed overkill.

[github-actions]

No obvious timing issues in HEAD=merge_statistics

Generated via commit abc4900

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	5 minutes and 17 seconds
Installing different package versions	11 minutes and 26 seconds
Running and plotting the test cases	4 minutes and 2 seconds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.03%. Comparing base (7c410cd) to head (abc4900).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7566   +/-   ##
=======================================
  Coverage   99.03%   99.03%           
=======================================
  Files          87       87           
  Lines       16930    16953   +23     
=======================================
+ Hits        16767    16790   +23     
  Misses        163      163           

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