pgsql: Improve the implementation of information_schema._pg_expandarray

Improve the implementation of information_schema._pg_expandarray().

This function was originally coded with a handmade expansion
of the array subscripts.  We can do it a little faster and far
more legibly today, by using unnest() WITH ORDINALITY.

While at it, let's apply the rowcount estimation support that exists
for the underlying unnest() function: reduce the default ROWS estimate
to 100 and attach array_unnest_support.  I'm not sure that
array_unnest_support can do anything useful today with the call sites
that exist in information_schema, but it can't hurt, and the existing
default rowcount of 1000 is surely much too high for any of these
cases.

The psql.sql regression script is using _pg_expandarray() as a
test case for \sf+.  While we could keep doing so, the new one-line
function body makes a poor test case for \sf+ row-numbering, so
switch it to print another information_schema function.

Discussion: https://postgr.es/m/1424303.1703355485@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/58054de2d0847c09ef091956f72ae5e9fb9a176e

Modified Files
--------------
src/backend/catalog/information_schema.sql |  7 ++----
src/backend/utils/adt/arrayfuncs.c         |  3 +++
src/include/catalog/catversion.h           |  2 +-
src/test/regress/expected/psql.out         | 40 ++++++++++++++++--------------
src/test/regress/sql/psql.sql              |  4 +--
5 files changed, 30 insertions(+), 26 deletions(-)