Move `mel-encoding-module-alist' to mime-def.el.

Move `mel-encoding-module-alist' to mime-def.el.
(mel-find-function): Use function `mel-find-function-from-obarray'.
Use `mel-define-backend' to define "7bit", "8bit" and "binary"; don't
define methods of "8bit" and "binary"; inherit methods from "7bit".