(MCodingSystem): Add const to an arg or decoder. (finish_decoding): Add const to an arg. (decode_coding_charset, decode_coding_utf_8) (decode_coding_utf_16, decode_coding_utf_32) (decode_coding_iso_2022, decode_coding_sjis) (mconv_define_coding): Likewise.
(MCodingSystem): Add const to an arg or decoder.
(finish_decoding): Add const to an arg.
(decode_coding_charset, decode_coding_utf_8)
(decode_coding_utf_16, decode_coding_utf_32)
(decode_coding_iso_2022, decode_coding_sjis)
(mconv_define_coding): Likewise.

xmirror/m17n-lib,xmirror/m17n-lib,xmirror/m17n-lib,xmirror/m17n-lib