Add Serializer._dict_class

Adds Serializer._dict_class and uses it instead of hard coded ReturnDict. Based on discussions in https://github.com/tomchristie/django-rest-framework/issues/1995. A package using the _dict_class can be found from: https://github.com/akaariai/drf-fast-serializer. Benchmark results also found from drf-fast-serializer.