(mail-header-entity): New macro.

(mail-header-entity): New macro.
(mail-header-set-entity): New macro.
(mail-header-set-field): New macro.
(mail-header-set-parsed-field): New macro.
(mail-header-subject): Use `mail-header-entity'.
(mail-header-set-subject): Use `mail-header-set-parsed-field'.
(mail-header-from): Use `mail-header-entity'.
(mail-header-set-from): Use `mail-header-set-parsed-field'.
(mail-header-date): Use `mime-fetch-field' and `mail-header-entity'.
(mail-header-set-date): Use `mail-header-set-field'.
(make-full-mail-header): Modify data structure to store Date to
mime-entity.
(make-mail-header): Use `make-full-mail-header'.