Define method functions of mel.

Define method functions of mel.
(mime-insert-encoded-file): Use `mel-define-method'; abolish
`quoted-printable-insert-encoded-file'.
(mime-write-decoded-region): Use `mel-define-method'; abolish
`quoted-printable-write-decoded-region'.
- Move `q-encoding-printable-char-p' and `q-encoding-encoded-length'
to mel.el.