4.1 comes with a more granular data definition hierarchy that will make it easier to
write more API definitions. This marked by the introduction of a new member known
as Entity. Entity only needs a pk() and get key(), as well
as member declarations to integrate fully.
Entity
- Useful for nested entities that don't have endpoints like LatestPrice.
 - Useful for non-REST style APIs like GraphQL.
 - Simplifies defining nested entities.
 
New hierarchy:
 SimpleRecord
      |
   Entity
      |
SimpleResource
      |
   Resource
Deprecations:
- Resource.getKey() -> Resource.key
 - Resource.getEntitySchema() -> Resource.asSchema()
 - Entity.define() -> override Entity.schema
 
Changes:
- Normalizr: top level key sent to getId is undefined not null
 - pk() now takes additional parent, and key optional args
 - pk() no longer accepts number return value (run .toString())
 
