Define method functions of mel.

Define method functions of mel.
(mime-encode-region): Use `mel-define-method-function'; abolish
`gzip64-encode-region'.
(mime-decode-region): Use `mel-define-method-function'; abolish
`gzip64-decode-region'.
(mime-encode-string): New method.
(mime-decode-string): New method.
(mime-insert-encoded-file): Use `mel-define-method'; abolish
`gzip64-insert-encoded-file'.
(mime-write-decoded-region): Use `mel-define-method'; abolish
`gzip64-write-decoded-region'.