[feature request] Support nested Queryable, Identifiable (and like those)

[KeenS]

Current diesel doesn't support nested Queryables nor Identifiables but I'd like to write code like below:

#[derive(Eq, PartialEq, Hash)]
#[derive(Queryable)]
struct UserId(pub i32);

#[derive(Queryable, Identifiable)]
struct User {
    // nested Queryable
    pub id: UserId,
    pub name: String,
}

#[derive(Queryable, Identifiable)]
#[table_name = "users"]
struct UserDetail {
    // nested Queryable, Identifiable.
    //  May need a certain attribute like `#[have_primary_key(id)]`
    pub user: User,
    pub email_address: String,
    pub screen_name: String,
}

fn find_user(cn: &SomeConnection, user_id: UserId) -> Result<User, Error> {
    //...
    // If `UserId` is `AsExpression` and can be use like `find(user_id)`, it's also awesome but not scope of this issue
    users.find(user_id.0)
        .select(((id), name))
        // or, allowing `select((id, name))` will be useful
        .get_result(cn)
 
}
fn find_user_detail(cn: &SomeConnection, user_id: UserId) -> Result<UserDetail, Error> {
    //...
    users.find(user_id.0)
        .select((((id), name), email_address, screen_name))
        // the same above. allowing `select((id, name, email_address, screen_name))` will be usefull
        .get_result(cn)
}

In summary,

• A struct is Queryable when its direct fields and fields of its child structs are isomorphic to the selecting sql fields
• A struct is Identifiable when it has a primary key or one of its child structs have a primary key

As far as Queryable is concerned, nested deriving itself is possible but impossible to get_result from any isomorphic sql types. (This may not the matter of Queryabel but the matter of FromSql and ToSql.)

Structs like struct UserId(i32); is very useful to avoid confusions when handling many table ids. Is it possibel to implement these features?