Add Postgres GENERATE_SERIES function

[BranislavLazic]

Adds GENERATE_SERIES function https://www.postgresql.org/docs/current/functions-srf.html
I could also add the GENERATE_SUBSCRIPTS function but it has to wait for array expressions #380.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.49%. Comparing base (6a0798e) to head (dfafd14).
Report is 7 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #397   +/-   ##
=======================================
  Coverage   90.49%   90.49%           
=======================================
  Files         125      125           
  Lines        7458     7463    +5     
=======================================
+ Hits         6749     6754    +5     
  Misses        550      550           
  Partials      159      159           

Flag	Coverage Δ
	90.49% <100.00%> (+<0.01%)	⬆️

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