(eword-encode-divide-into-charset-words): Use `aref' instead of

(eword-encode-divide-into-charset-words): Use `aref' instead of
`sref'.
(ew-encode-rword-1): Use `1+' instead of `char-next-index'.
(eword-encode-phrase-to-rword-list): Use `find-charset-string' instead
of `find-non-ascii-charset-string'.
(eword-encode-addr-seq-to-rword-list): Don't use `butlast'.
(eword-encode-header): Use `find-charset-region' instead of
`find-non-ascii-charset-string'.