Move `mime-entity-implementation-alist' to mime-def.el.

Move `mime-entity-implementation-alist' to mime-def.el.
(mime-find-function): New implementation.
(mime-entity-cooked-p): Use `mime-entity-send'.
(mime-entity-point-min): Change message to `point-min'.
(mime-entity-point-max): Change message to `point-max'.
(mime-entity-header-start): Change message to `header-start'.
(mime-entity-header-end): Change message to `header-end'.
(mime-entity-body-start): Change message to `body-start'.
(mime-entity-body-end): Change message to `body-end'.