support for custom aggregates

[vekonypeter]

for example:

create or replace function array_union_step (s anyarray, n anyarray) returns anyarray
   as $$ select s || n; $$
   language sql immutable leakproof parallel safe;

create or replace function array_union_final (s anyarray) returns anyarray
  as $$
    select array_agg(i order by i) from (
      select distinct unnest(x) as i from (values(s)) as v(x)
    ) as w where i is not null;
  $$
  language sql immutable leakproof parallel safe;

create or replace aggregate array_union (anyarray) (
  sfunc = array_union_step,
  stype = anyarray,
  finalfunc = array_union_final,
  initcond = '{}',
  parallel = safe
);

comment on function array_union is
    E'Aggregate function to compute the union of multiple arrays';

the two "helper" functions (array_union_step and array_union_final) are recognized and diffed correctly, but the array_union aggregate is not, the tool just logs the fact that the statement is skipped.