For discussion:
Consider evaluating association in serializer context
That way, associations are really just anything that
can be conditionally included. They no longer
have to actually be methods on the object or serializer.
e.g.
```diff
has_many :comments do
- last(1)
+ Comment.active.for_serialization(object).last(1)
end
```
* Move all associations related code from Serializer class to Associations module
* Introduce Reflection class hierarchy
* Introduce Association class
* Rid off Serializer#each_association
* Introduce Serializer#associations enumerator
