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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All modified and coverable lines are covered by tests ✅

Project coverage is 90.49%. Comparing base (6a0798e) to head (dfafd14).
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@@           Coverage Diff           @@
##           master     #397   +/-   ##
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  Coverage   90.49%   90.49%           
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  Files         125      125           
  Lines        7458     7463    +5     
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+ Hits         6749     6754    +5     
  Misses        550      550           
  Partials      159      159           

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[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.