(TopLevel): Require `mmgeneric' always.

(TopLevel): Require `mmgeneric' always.
(mmgnus-entiry::mime-entity-fetch-field): Define method.
(mmgnus-entiry::mime-entity-body): Do not error.
(mmgnus-entiry::mime-insert-text-content): Define method.
(mmgnus-entiry::mime-insert-entity-body): Ditto.
(mime-gnus-entity::mime-entity-fetch-field): Ditto.
(mime-gnus-entity::mime-entity-body): Ditto.
(mime-gnus-entity::mime-entity-content): Ditto.
(mime-gnus-entity::mime-insert-entity-content): Ditto.
(mime-gnus-entity::mime-insert-text-content): Ditto.
(mime-gnus-entity::mime-write-entity-content): Ditto.
(mime-gnus-entity::mime-insert-entity): Ditto.
(mime-gnus-entity::mime-write-entity): Ditto.
(mime-gnus-entity::mime-insert-entity-body): Ditto.
(mime-gnus-entity::mime-write-entity-body): Ditto.