[POC] Allow flattening of subquery with a link to upper query

Hi,

One of the most annoying things in the planner for me is unnesting of
correlated queries [1]. A number papers on this subject were published
starting 1980s, but only trivial optimizations exists in the Core. It
means a lack of performance, especially when we use foreign tables in
subquery.
In the patch I'm trying to propose a sort of sketch of solution.

Before flattening procedure we just look through the quals of subquery,
pull to the upper level OpExpr's containing variables from the upper
relation and replace their positions in the quals with true expression.
Further, the flattening machinery works as usual.

This patch is dedicated to simplest variant of correlated queries -
without aggregate functions in the target list. It passes regression
tests and contains some additional tests to demonstrate achievements.

I'd like to get critics on the approach.

[1] Kim, Won. “On optimizing an SQL-like nested query.” ACM Trans.
Database Syst. 7 (1982): 443-469.

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Regards
Andrey Lepikhov
Postgres Professional