Postgres: add a native reverse(bytea) function

[bitjson]

It would be nice if Postgres supported a native reverse(bytea) function. Currently we're manually reversing byteas with this plpgsql function (which is presumably somewhat slower than a native option?):

chaingraph/images/hasura/hasura-data/migrations/default/1616195337538_init/up.sql

Lines 719 to 732 in cbebede

	CREATE OR REPLACE FUNCTION reverse_bytea(source_bytes bytea) RETURNS bytea
	LANGUAGE plpgsql IMMUTABLE
	AS $$
	DECLARE
	last_source_index integer := octet_length(source_bytes) - 1;
	reversed bytea := source_bytes; -- set result to proper length
	BEGIN
	FOR i IN 0..last_source_index LOOP
	reversed := set_byte(reversed, i, get_byte(source_bytes, last_source_index - i));
	END LOOP;
	RETURN reversed;
	END;
	$$;
	COMMENT ON FUNCTION reverse_bytea(bytea) IS 'Reverse the byte order of a bytea.';

There doesn't seem to be a good reason why Postgres wouldn't support reversing bytea types natively. We might want to submit a patch. Looks like we'd need to add it to src/backend/utils/adt/varlena.c (see text_reverse).