(CHAR_STRING_ADVANCE_NO_UNIFY)

(CHAR_STRING_ADVANCE_NO_UNIFY)
(STRING_CHAR_ADVANCE_NO_UNIFY): New macros.
(coding_alloc_by_making_gap): Fix the way to preserve data in the
gap.
(alloc_destination): Fix the 2nd arg to
coding_alloc_by_making_gap.
(encode_coding_utf_8): Use CHAR_STRING_ADVANCE_NO_UNIFY instead of
CHAR_STRING_ADVANCE.
(produce_chars): Fix for the case that the source and the
destination are the same buffer.  Use CHAR_STRING_ADVANCE_NO_UNIFY
instead of CHAR_STRING_ADVANCE.
(consume_chars): Use STRING_CHAR_ADVANCE_NO_UNIFY instead of
STRING_CHAR_ADVANCE.