natural join using X[on=Y], closes #3621

[jangorecki]

closes #3621

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5318695). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #3732   +/-   ##
========================================
  Coverage          ?   99.4%           
========================================
  Files             ?      71           
  Lines             ?   13159           
  Branches          ?       0           
========================================
  Hits              ?   13081           
  Misses            ?      78           
  Partials          ?       0

Impacted Files	Coverage Δ
R/data.table.R	99.94% <100%> (ø)

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