Fix bug #64023 (__toString() & SplFileInfo)

Defining a __toString() method was having no effect when concatenating
the object. This was because the cast_object() handler would ignore
__toString().

Using echo() directly would actually use __toString(), but this was a
bug: the ECHO handler would try zend_std_cast_object_tostring() before
cast_object(), but cast_object() should have priority as
zend_std_cast_object_tostring() assumes an object with a
zend_class_entry.
NEWS for bug #64023 (see aa0adce)