Define method functions of mel.

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.