Avoid copying when changing the type of an indexed column

It was the intention of MDEV-15564 to allow ALGORITHM=NOCOPY for changing the character set or collation of an indexed column. Currently, such operations will degrade to ALGORITHM=COPY:

Please see the test innodb.instant_alter_charset. We would expect results like

ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: ADD INDEX. Try ALGORITHM=NOCOPY

but instead, we currently get the following:

ERROR 0A000: ALGORITHM=INSTANT is not supported. Reason: Cannot change column type INPLACE. Try ALGORITHM=COPY

I believe that this is due to the following in Field_str::is_equal() and Field_varstring::is_equal():

  if (!Type_handler::Charsets_are_compatible(field_charset, new_field->charset,

					     part_of_a_key))

    return IS_EQUAL_NO;

Maybe we need variants of IS_EQUAL_PACK_LENGTH and IS_EQUAL_PACK_LENGTH_EXT that indicate that the column is indexed, and therefore the SQL layer must request the indexes to be dropped and added.