(mime-entity-implementation-alist): New variable.

(mime-entity-implementation-alist): New variable.
(mime-find-function): New function.
(mime-open-entity): New function.
(mime-entity-function): New function.
(mime-entity-cooked-p): New function.
(mime-entity-buffer): Use backend-module.
(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.