(base64-encode-region): Define directly (abolish

(base64-encode-region): Define directly (abolish
`base64-dl-encode-region').
(base64-decode-region): Define directly (abolish
`base64-dl-decode-region').
Define method functions of mel.
(mime-insert-encoded-file): Use `mel-define-method'; abolish
`base64-insert-encoded-file'; don't use external encoder.
(mime-write-decoded-region): Use `mel-define-method'; abolish
`base64-write-decoded-region'; don't use external decoder.
- Move `base64-encoded-length' to mel.el.