Add PureXML.Parsing.TextReader (via PureXML.reader), a pull cursor over the
streaming core. read() advances node by node, exposing nodeKind, name,
value, depth, attributes, and isEmptyElement. A childless element is
reported once as an empty element, normalizing <a/> and <a></a>. The
documentType accessor surfaces the DTD read so far as the validation hook.
Closes#33
