Implement a dereferencing UDF function

[hatyo]

It would be useful for certain scenarios to implement a dereferencing UDF offers a shortcut for dereferencing a nested field:

create type as struct st2(w bigint, x bigint)
create type as struct st3(u st2, v st1)
create type as struct st4(s bigint, t bigint)
create table nested(id bigint, q st4, r st3, primary key(q.s, r.u.w))
create function fun(in x type st3) RETURNS ST2 AS x.u
select fun(r) from nested