(mime-entity-send): New function.

(mime-entity-buffer): Use `mime-entity-send'.
(mime-entity-point-min): Likewise.
(mime-entity-point-max): Likewise.
(mime-entity-header-start): Likewise.
(mime-entity-header-end): Likewise.
(mime-entity-body-start): Likewise.
(mime-entity-body-end): Likewise.
(mime-fetch-field): Likewise.
(mime-entity-content): Use implementation of mm-backend.
(mime-write-entity-content): Likewise.