(XmlToken, bool) where the bool is true when a token
was successfully produced, and false when more data is needed.
XmlLexerResult tryNext() @safeAttempts to produce the next token from the buffered input.
(XmlToken, bool) where the bool is true when a token
was successfully produced, and false when more data is needed.
XmlException on lexical errors, or UNEXPECTED_EOF when the stream
is marked as ended but an incomplete token remains.