Virtual generated columns

Here is a patch set to implement virtual generated columns.

Some history first: The original development of generated columns was
discussed in [0]. It started with virtual columns, then added stored
columns. Before the release of PG12, it was decided that only stored
columns were ready, so I cut out virtual columns, and stored generated
columns shipped with PG12, which is where we are today.

Virtual generated columns are occasionally requested still, and it's a
bit of unfinished business for me, too, so I started to resurrect it.
What I did here first was to basically reverse interdiff the patches
where I cut out virtual generated columns above (this was between
patches v8 and v9 in [0]) and clean that up and make it work again.

One thing that I needed to decide was how to organize the tests for
this. The original patch series had both stored and virtual tests in
the same test file src/test/regress/sql/generated.sql. As that file has
grown, I think it would have been a mess to weave another whole set of
tests into that. So instead I figured I'd make two separate test files

src/test/regress/sql/generated_stored.sql (renamed from current)
src/test/regress/sql/generated_virtual.sql

and kind of try to keep them aligned, similar to how the various
collate* tests are handled. So I put that renaming in as preparatory
patches. And there are also some other preparatory cleanup patches that
I'm including.

The main feature patch (0005 here) generally works but has a number of
open corner cases that need to be thought about and/or fixed, many of
which are marked in the code or the tests. I'll continue working on
that. But I wanted to see if I can get some feedback on the test
structure, so I don't have to keep changing it around later.

[0]:
https://www.postgresql.org/message-id/flat/b151f851-4019-bdb1-699e-ebab07d2f40a(at)2ndquadrant(dot)com