Consider evaluating association in serializer context

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
```

test/fixtures/poro.rb

@@ -33,6 +33,7 @@ end
 class ProfileSerializer < ActiveModel::Serializer
   attributes :name, :description
 
+  # TODO: is this used anywhere?
   def arguments_passed_in?
     instance_options[:my_options] == :accessible
   end
@@ -75,6 +76,7 @@ PostSerializer = Class.new(ActiveModel::Serializer) do
     Blog.new(id: 999, name: 'Custom blog')
   end
 
+  # TODO: is this used anywhere?
   def custom_options
     instance_options
   end

test/serializers/associations_test.rb

@@ -129,7 +129,7 @@ module ActiveModel
       class InlineAssociationTestPostSerializer < ActiveModel::Serializer
         has_many :comments
         has_many :comments, key: :last_comments do
-          last(1)
+          object.comments.last(1)
         end
       end