Add PureXML.Parsing.SAXHandler (a struct of optional callbacks) and
PureXML.parse(_:sax:), which drives the handler from the streaming event core,
delivering start/end document, start/end element, characters, CDATA, comment,
and processing-instruction callbacks. Element names are namespace-resolved.
Closes the SAX2 epic (#19). Reimplemented from the libxml2 SAX2 model; no
upstream code copied. 5 tests; 84 total.
