Nested objects extract

This is a draft of a nested object extract feature by assigning the object => hydrator key and value pair to renderer/hydrators map.
This change is a potential backwards compatibility break. Currently, `fromResource()` will return an empty array if no links are defined, giving:

```json
{
    "_links": []
}
```
(or possibly `{}` as the value). One unit test fails because of the change.

That said, semantically, if there are no links, we shouldn't likely inject the object.