Subqueries

As discussed earlier, we heavily need subqueries (or EXTERNAL JOIN) like:

```
let sub l = <:view< group { tags = array_agg[t.name] } | t in $tags$; something with l and t >> in
Query.view db <:view< { tags = one ($sub$ l); l.id } | l in $links$ >>
```

So I'm making this issue to discuss and have feedbacks. I'll be glad to help if you need.
