dict_index_t::add_field(): Replaces dict_mem_index_add_field().
      dict_index_t::get_field(): Replaces dict_index_get_nth_field().
      dict_index_t::get_col(): Replaces dict_index_get_nth_col().
      dict_index_t::get_col_no(): Replaces dict_index_get_nth_col_no().
      dict_index_t::get_col_pos(): Replaces dict_index_get_nth_col_pos()
   and dict_index_get_nth_col_or_prefix_pos().
      dict_index_t::get_sys_col_pos(): Replaces dict_index_get_sys_col_pos().

RB#13487
Reviewed by Kevin Lewis