(concat): Handle 8-bit characters correctly.

(concat): Handle 8-bit characters correctly.
(Fstring_as_unibyte): Be sure to make all 8-bit characters in
unibyte in the result.
(Fstring_as_multibyte): Be sure to make all 8-bit characters in
valid multibyte form in the result.
(map_char_table): Use MAKE_CHAR instead of MAKE_NON_ASCII_CHAR.
(Fbase64_encode_region, Fbase64_encode_string): If base64_encode_1
return -1, signal an error.
(base64_encode_1): New arg MULTIBYTE.  Get each character by
CHAR_STRING_AND_LENGTH if MULTIBYTE is nonzero.  If a multibyte
character is found, return -1.
(Fbase64_decode_region): Delete codes for handling byte-combining.
Treat each decoded byte as a unibyte character.
(Fbase64_decode_string): Return unibyte string.
(Fcompare_strings, concat, string_byte_to_char): Use
FETCH_STRING_CHAR_ADVANCE_NO_CHECK instead off
FETCH_STRING_CHAR_ADVANCE.
(Fstring_lessp): Use FETCH_STRING_CHAR_ADVANCE unconditionally.
(mapcar1): If SEQ is string, always use FETCH_STRING_CHAR_ADVANCE.