Move the code from Field_str::is_equal() to Field_string::is_equal()

Field_str in an abstract class with this hierarchy:

Field_str                        [*]

  Field_longstr

    Field_string

    Field_varstring              [*]

      Field_varstring_compressed

    Field_blob                   [*]

      Field_blob_compressed

      Field_geom                 [*]

  Field_null

    Field_row

  Field_enum                     [*]

    Field_set

[*] marks classes that their own virtual is_equal() implementation.

Most Field_str descendants completeley override is_equal() and do not share the implementation on the Field_str level.

Only Field_string and Field_null share Field_str::is_equal().
If we take a look inside Field_str::is_equal(), it actually implements behavior for CHAR.

Under term of this task we'll:

• Move the code from Field_str::is_equal() to Field_string::is_equal()
• Override Field_null::is_equal() to a simple implementation. The current shared implementation is too exsessive for Field_null, and is going to be even more complex under terms of MDEV-17301. It can be simplified to something like this:

  uint Field_null::is_equal(Create_field *new_field)

  {

  DBUG_ASSERT(!compression_method());

  return new_field->type_handler() == type_handler() &&

  new_field->charset == field_charset &&

  new_field->length == max_display_length();

  }

• Remove the default unused implementation Field::is_equal() and define it as a zero pointer:

  virtual uint is_equal(Create_field *new_field)= 0;