Add the Posts example to illustrate a use for $lookup

[drpicox]

This adds an example based on the recommendations for a redux state shape:

• https://redux.js.org/recipes/structuring-reducers/normalizing-state-shape/

I have added this because I believe that a good example of $lookup should be beneficial.
The added query is:

(
    $comment := function($cid) {( comments.byId.$lookup($cid) )};
    $post := function($pid) {( posts.byId.$lookup($pid) )};
    posts.allIds{$: $post($).comments.$comment($)}
)

[andrew-coleman]

This is good, but I would have used the following expression for the same effect:

posts.allIds@$P{
    $P: byId.$lookup($P).comments.$lookup($$.comments.byId, $)
}

alternatively, if you can assume that the list of post IDs in posts.allIds is the same as the entries in posts.byId (and I'm happy if you can't assume this) then you could simplify it:

posts.byId.*{
    id: comments.$lookup($$.comments.byId, $)
}