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]

• HEAD=merge_statistics much slower for as.data.table.array improved in #7019
• HEAD=merge_statistics much slower for transform improved in #5493
  

Generated via commit ea5741b

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

Task	Duration
R setup and installing dependencies	6 minutes and 37 seconds
Installing different package versions	12 minutes and 18 seconds
Running and plotting the test cases	6 minutes and 48 seconds

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (895067d) to head (ea5741b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7566   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          88       88           
  Lines       17266    17289   +23     
=======================================
+ Hits        17097    17120   +23     
  Misses        169      169           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jangorecki]

Afair we have anti join in mergelist, should that be plugged into using this one now?
Uh no, this is only for verbose