xml: Add classes to help testing a SAX parser.

Will be used to test SAXophoNit (A SAX parser in Nit).

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
xml: Introduce SAXophoNit, a SAX processor in Nit.

For the moment, this implementation is mostly non-compliant, but it
works with most common XML documents.

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>