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

src/backend/utils/adt/arrayfuncs.c

@@ -6317,6 +6317,9 @@ array_unnest(PG_FUNCTION_ARGS)
 
 /*
  * Planner support function for array_unnest(anyarray)
+ *
+ * Note: this is now also used for information_schema._pg_expandarray(),
+ * which is simply a wrapper around array_unnest().
  */
 Datum
 array_unnest_support(PG_FUNCTION_ARGS)