Avoid some table rewrites for ALTER TABLE .. SET DATA TYPE array coerce

Hi.

Context: https://git.postgresql.org/cgit/postgresql.git/commit/?id=3c5926301aea476025f118159688a6a88b2738bc
Also see build_coercion_expression, COERCION_PATH_ARRAYCOERCE handling.

It's doable to skip a table rewrite when changing a column's data type from
one array type to another. We just need some logic to handle
ArrayCoerceExpr within ATColumnChangeRequiresRewrite.

ArrayCoerceExpr have two node expression, ``arg`` and ``elemexpr``, therefore
ATColumnChangeRequiresRewrite, the old single ``FOR(;;)`` loop is not enough, we
need recursive walking through ArrayCoerceExpr->arg and
ArrayCoerceExpr->elemexpr.

Please see the attached POC.

DEMO:
create table rewriteme (id serial primary key, foo float, bar timestamptz);
begin;
alter table rewriteme add column ttz_arr timestamptz[];
set timezone to 'UTC';
alter table rewriteme alter column bar type timestamp;
---- no table rewrite with PATCH, require table rewrite in HEAD
alter table rewriteme alter column ttz_arr type timestamp[];
rollback;

CREATE DOMAIN domain1 as INT;
CREATE TABLE test_set_col_type(a INT[]);

---- no table rewrite with PATCH, require table rewrite in HEAD
ALTER TABLE test_set_col_type ALTER COLUMN a SET DATA TYPE domain1[];

---- no table rewrite with PATCH, require table rewrite in HEAD
ALTER TABLE test_set_col_type ALTER COLUMN a SET DATA TYPE INT[];

--
jian
https://www.enterprisedb.com/