Under terms of this task we'll add a new method into the class Field, for symmetry with the class Item:
We'll also change methods Field::type(), Field::real_type(), Field::cmp_type(), Field::result_type() to be non-virtual, they will use the corresponding type_handler() methods as follows:
We'll also move the virtual method Field::cast_to_int_type_handler() from Field to Type_handler.
We won't change or move Field::binlog_type() for now. It will be done in a separate task.