Added WP_Field_Base->input_feature()
Renamed WP_Field_Feature_Base->html_element to be just element
Added get_input_html() and input_feature() to WP_Field_View_Base plus
Changed WP_Field_View_Base->get_features_html() to call  $this->get_input_html() instead of  $feature->get_feature_html() for input feature.
Added register_field_type() and register_autoload_class()
Added WP_Metadata::register_autoload_class() and modified to just class static vs. local static, and prefixed with underscore to imply hidden.
Added static modified to _enqueue_admin_styles().