Use real callables for XML handlers

As per PHP docs, the XML functions also accept just a method name as a string,
even though it is not actually a callable:

    Note: Instead of a function name, an array containing an object reference and a method name can also be supplied.

PHPStan level 5 will complain though:

    Parameter simplepie#2 $handler of function xml_set_character_data_handler expects callable(): mixed, 'cdata' given.

Let’s use proper callables.

src/Parser.php

@@ -111,8 +111,8 @@ public function parse(&$data, $encoding, $url = '')
             xml_parser_set_option($xml, XML_OPTION_SKIP_WHITE, 1);
             xml_parser_set_option($xml, XML_OPTION_CASE_FOLDING, 0);
             xml_set_object($xml, $this);
-            xml_set_character_data_handler($xml, 'cdata');
-            xml_set_element_handler($xml, 'tag_open', 'tag_close');
+            xml_set_character_data_handler($xml, [$this, 'cdata']);
+            xml_set_element_handler($xml, [$this, 'tag_open'], [$this, 'tag_close']);
 
             // Parse!
             $wrapper = @is_writable(sys_get_temp_dir()) ? 'php://temp' : 'php://memory';