PostgREST compatible computed relationships

[olirice]

What kind of change does this PR introduce?

WIP Adding support for PostgREST style computed relationships

For example:

create or replace function public.computed_books(rec public.account)
	returns setof book
	immutable
	strict
	language sql
as $$
	select * from book where book.author_id = rec.id
$$;

TODO:

• tests
• handle anonymous tables in some way (make them work or raise an error)
• handle returns setoff <entity> rows 1 as to-one not to-many
• document preference for returns setoff <entity> rows 1 for inlining
• update the computed relationship docs

resolves #357