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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.49%. Comparing base (6a0798e) to head (dfafd14).
Report is 29 commits behind head on master.

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%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

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

[go-jet]

Note that generate_series can be used as table source and expression:

SELECT * 
FROM generate_series(2,4); -- table source

SELECT generate_series(2,4); -- expression

With this PR the expression use case would be supported, but not the table source. Now to support table source it is a bit more work. I'm wondering if is it worth supporting the table source at all. At least to my knowledge everything that can be done with generate_series as a table source can be also done as an expression.

[go-jet]

I guess we can go with just expression form, and revisit the other form if needed.