Split Item_func_hex::val_str_ascii() into virtual methods in Type_handler

Item_func_hex::val_str_ascii() has a few tests on args[0]->result_type().

   if (args[0]->result_type() == REAL_RESULT ||

       args[0]->result_type() == DECIMAL_RESULT)

We'll move the data type specific code to the corresponding Type_handler_xxx, and rewrite Item_func_hex::val_str_ascii() to call a virtual method in Type_handler, e.g. about like this:

 String *val_str_ascii(String *str)

 {

   return args[0]->type_handler()->Item_func_hex_val_str_ascii(this, str);

 }

Note, the value of args[0]->type_handler() can be cached at fix_length_and_dec() time, to have fewer virtual calls at run time:

 String *val_str_ascii(String *str)

 {

   return m_handler->Item_func_hex_val_str_ascii(this, str);

 }