Add Field::type_handler()

Under terms of this task we'll add a new method into the class Field, for symmetry with the class Item:

virtual const Type_handler *type_handler() const;

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:

enum_field_types type() const

{

  return type_handler()->field_type();

}

enum_field_types real_type() const

{

  return type_handler()->real_field_type();

}

Item_result result_type () const

{

  return type_handler()->result_type();

}

Item_result cmp_type () const

{

  return type_handler()->cmp_type();

}

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.