(mm-util): Require.

(mm-util): Require.
(utf7-direct-encoding-chars, utf7-imap-direct-encoding-chars):
Defconst, not defvar.
(utf7-utf-16-coding-system): New.
(utf7-encode-internal): Hoist concat out of loop.
(utf7-fragment-encode): Use mm-with-unibyte-current-buffer.
(utf7-get-u16char-converter) [utf7-utf-16-coding-system]: New
case.
(utf7-latin1-u16-char-converter): Encode the region.
(utf7-u16-latin1-char-converter): Decode the region.
(utf7-encode, utf7-decode): Fix multibyteness.