Suggestions:
* Deprecate _field_types in favor of __annotations__.
* Convert __annotations__ from OrderedDict to a regular dict.

This will make the API cleaner. The first one will also remove a difference between NamedTuple and namedtuple().  The second is consistent with the decision to convert _asdict() to a regular dictionary since OrderedDict is no longer necessary or desirable. The second will also make the signature of __annotations__ match that from other classes.