For JSONAPI, `include_data` currently means, "should we populate the
'data'" key for this relationship. Current options are true/false.
This adds the `:if_sideloaded` option. This means "only
populate the 'data' key when we are sideloading this relationship." This
is because 'data' is often only relevant to sideloading, and causes a
database hit.
Addresses https://github.com/rails-api/active_model_serializers/issues/1555
* Make assocations asserts easier to understand
* Refactor Association into Field like everything else
* Make assocation serializer/links/meta lazier
* Push association deeper into relationship
* Simplify association usage in relationships
* Better naming of reflection parent serializer
* Easier to read association method
