(base64-token-regexp): New constant (moved from eword-decode.el).

(base64-token-regexp): New constant (moved from eword-decode.el).
(base64-token-padding-regexp): Likewise.
(B-encoded-text-regexp): New constant (moved from eword-decode.el, and
renamed from `eword-B-encoded-text-regexp').
(Q-encoded-text-regexp): New constant (moved from eword-decode.el, and
renamed from `eword-Q-encoded-text-regexp'.
(mm-arglist-to-arguments): New function (moved from mime.el).
(mel-define-method): New macro.
(mel-define-method-function): New macro.
(mel-define-function): New macro.