feat: add one to one relationship for embedding

BREAKING CHANGE: For the cases where one to one relationships are
detected, json objects will be returned instead of json arrays of length
1.

If you wish to override this behavior, you can use computed
relationships to return arrays again.

test/spec/fixtures/schema.sql

@@ -2400,7 +2400,7 @@ CREATE TABLE contact (
 
 CREATE TABLE clientinfo (
   id serial primary key
-, clientid int unique references client(id)
+, clientid int references client(id)
 , other text
 );
 
@@ -2753,3 +2753,76 @@ $$ LANGUAGE sql STABLE ROWS 1;
 CREATE FUNCTION test.videogames(test.designers) RETURNS SETOF test.videogames AS $$
   SELECT * FROM test.videogames WHERE FALSE;
 $$ LANGUAGE sql STABLE;
+
+CREATE TABLE test.students(
+  id int
+, code text
+, name text
+, primary key(id, code)
+);
+
+CREATE TABLE test.students_info(
+  id int
+, code text
+, address text
+, primary key(id, code)
+, foreign key (id, code) references test.students(id, code) on delete cascade
+);
+
+CREATE TABLE test.country(
+  id int primary key
+, name text
+);
+
+CREATE TABLE test.capital(
+  id int primary key
+, name text
+, country_id int unique
+, foreign key (country_id) references test.country(id)
+);
+
+CREATE FUNCTION test.allcountries() RETURNS SETOF test.country AS $$
+  SELECT * FROM test.country;
+$$ LANGUAGE sql STABLE;
+
+CREATE FUNCTION test.allcapitals() RETURNS SETOF test.capital AS $$
+  SELECT * FROM test.capital;
+$$ LANGUAGE sql STABLE;
+
+create view students_view as
+select * from students;
+
+create view students_info_view as
+select * from students_info;
+
+create table test.second (
+  id int primary key,
+  name text
+);
+
+create table test.first (
+  id int primary key,
+  name text,
+  second_id_1 int references test.second unique,
+  second_id_2 int references test.second unique
+);
+
+create table test.second_1 (
+  id int primary key,
+  name text
+);
+
+create table test.first_1 (
+  id int primary key,
+  name text,
+  second_id_1 int references test.second unique,
+  second_id_2 int references test.second unique
+);
+
+CREATE FUNCTION test.second_1(test.first_1) RETURNS SETOF test.second_1 AS $$
+  SELECT * FROM test.second_1 WHERE id = $1.second_id_1;
+$$ LANGUAGE sql STABLE ROWS 1;
+
+CREATE FUNCTION test.first_1(test.second_1) RETURNS SETOF test.first_1 AS $$
+  SELECT * FROM test.first_1 WHERE second_id_1 = $1.id;
+$$ LANGUAGE sql STABLE ROWS 1;