Adds an alpha-stage prototype: FastQuery
FastQuery enables the use of `.values` querysets, to bypass the overhead involved with marshalling Django models. This is implemented via a queryset-compatible API, to attempt to make the distinction between model-based querysets and dictionary-based querysets invisible to the serialization layer.
This version also brings some improvements around caching of `Field`s.
These changes are undocumented and untested, and should be treated as unstable. They can only be used by opting into them: for `FastQuery`, this means using the `FastDynamicFilterBackend`; for fields caching, this means setting the `ENABLE_FIELDS_CACHE` setting to `True` and setting a class variable (`ENABLE_FIELDS_CACHE` as well) on individual serializers.