MDEV-6666 Malformed result for CONCAT(utf8_column, binary_string)

Item_static_string_func::safe_charset_converter() and 
Item_hex_string::safe_charset_converter() did not
handle character sets with mbminlen>1 properly, as well as
did not handle conversion from binary to multi-byte well.

Introducing Item::const_charset_converter(), to reuse it in a number
of Item_*::safe_charset_converter().