Make SimpleXMLElement a RecursiveIterator

Make SimpleXMLElement a RecursiveIterator

Context: https://externals.io/message/108789

This essentially moves the functionality of SimpleXMLIterator into
SimpleXMLElement, and makes SimpleXMLIterator a no-op extension.

Ideally SimpleXMLElement would be an IteratorAggregate, whose
getIterator() method returns SimpleXMLIterator. However, because
SimpleXMLIterator extends SimpleXMLElement (and code depends on
this in non-trivial ways), this is not possible.

The only way to not keep SimpleXMLElement as a magic Traversable
(that implements neither Iterator nor IteratorAggregate) is to
move the SimpleXMLIterator functionality into it.

Closes GH-5234.