Remove parse method with callback. Moved next() method to Iterator impl

- XmlParser no longer supports function callbacks, because they are part of
  push Xml API
- XmlParser now uses next() method to iterate over the XML stream
Fix test to work with new signature; Add parse_char

- Fix the breakage caused by previous commit in which next was added to
  iterator implementation.
- parse_char is the new function that determines which switches states
  depending on given char.