(utf-16-decode-ucs): Look up

(utf-16-decode-ucs): Look up
utf-subst-table-for-decode.  Fix for the case that the looking up
succeeds.
(ccl-decode-mule-utf-16-le): Translate characters by
utf-translation-table-for-decode.
(ccl-decode-mule-utf-16-be): Likewise.
(ccl-encode-mule-utf-16-le): Look up utf-subst-table-for-encode
at first.  Translate characters by
utf-translation-table-for-encode.
(ccl-encode-mule-utf-16-be): Likewise.
(mule-utf-16-le, mule-utf-16-be): Add `dependency' property.