Esqueleto does not allow access to sub selects

Not sure if you are looking for issues like this as inspiration/todo. I'll pop this in esqueleto if it doesn't belong here.

Esqueleto's `sub_select` has this type `query (expr (Value a)) -> expr (Value a)`. In raw sql I often want to utilize columns from a sub select for purposes other than returning (order, group, etc).

So you'd need something to unwrap the `Value`, like `expr (Value a) -> expr a`, since `^.` has type `expr (Entity val) -> EntityField val typ -> expr (Value typ) `

Currently this is not possible.