Have PgSelectStep leverage unary steps

[benjie]

Relates to #1973; PgSelectStep and related classes should be able to addUnaryDependency for first/etc and thus not need to do such complex SQL for first/last/cursor/etc. Should be able to use this to eliminate some eval calls also?

[benjie]

If the dependency is a unary then instead of:

select __organization_memberships_result__.*
from (select 0 as idx, $1::"int4" as "id0") as __organization_memberships_identifiers__,
lateral (
  select
    __organization_memberships__."organization_id"::text as "0",
    __organization_memberships__."id"::text as "1",
    __organization_memberships_identifiers__.idx as "2"
  from "app_public"."organization_memberships" as __organization_memberships__
  where (
    __organization_memberships__."user_id" = __organization_memberships_identifiers__."id0"
  )
  order by __organization_memberships__."id" asc
) as __organization_memberships_result__;

we can do

  select
    __organization_memberships__."organization_id"::text as "0",
    __organization_memberships__."id"::text as "1"
  from "app_public"."organization_memberships" as __organization_memberships__
  where (
    __organization_memberships__."user_id" = $1::"int4"
  )
  order by __organization_memberships__."id" asc;