NFC: refactor Field::is_equal() and related stuff

Make Field::is_equal() const and return bool as it's a naturally fitting
type for it. Also it's agrument was narrowed to Column_definition.

InnoDB can change type of some columns by itself. InnoDB-specific code used to
reside in Field_xxx:is_equal() methods. Now engine-specific stuff was
moved to a virtual methods of handler::can_convert{string,varstring,blob,geom}.
These methods are called by Field::can_be_converted_by_engine() which is a
double dispatch pattern.

Some InnoDB-specific code still resides in compare_keys_but_name(). It should
be moved from here someday to handler::compare_key_parts(...) or similar.

IS_EQUAL_WITH_REINTERPRET_COMPATIBLE_CHARSET
IS_EQUAL_WITH_REINTERPRET_COMPATIBLE_CHARSET_BUT_COLLATE: both was removed

IS_EQUAL_NO, IS_EQUAL_YES are not needed now and should be removed
along with deprecated handler::check_if_incompatible_data().

HA_EXTENDED_TYPES_CONVERSION: was removed as such logic is not needed now by
server code.

ALTER_COLUMN_EQUAL_PACK_LENGTH: was renamed to a more generic
ALTER_COLUMN_TYPE_CHANGE_BY_ENGINE

sql/sql_type.cc

@@ -8367,6 +8367,5 @@ Charset::eq_collation_specific_names(CHARSET_INFO *cs) const
 {
   LEX_CSTRING name0= collation_specific_name();
   LEX_CSTRING name1= Charset(cs).collation_specific_name();
-  /* Empty collations are not equal */
   return name0.length && !cmp(&name0, &name1);
 }