(eword-field-encoding-method-alist): Moved to mime-def.el and renamed

(eword-field-encoding-method-alist): Moved to mime-def.el and renamed
to `mime-field-encoding-method-alist'.
(mime-header-charset-encoding-alist): Renamed from
`eword-charset-encoding-alist'.
(mime-header-default-charset-encoding): New variable.
(ew-find-charset-rule): Use `mime-header-default-charset-encoding'.
(eword-in-subject-p): Declare as obsolete function.
(mime-encode-header-in-buffer): Renamed from `eword-encode-header';
declare `eword-encode-header' as obsolete alias.