Macro relation 

macro_rules! relation {
    ($($model:ident)::+ . $field:ident) => { ... };
}

Build a RelationLens from a model and one of its relation fields.

relation!(Post.author) is the include Post::fields().author() paired with |post| &post.author.

let author: RelationLens<Post, Deferred<Author>> = relation!(Post.author);
let comments: RelationLens<Post, Deferred<Vec<Comment>>> = relation!(Post.comments);
let qualified = relation!(crate::blog::Post.author);