Minor fix

- Add docs to util::is_whitespace
- Add symbol equal to XmlToken enum
- Remove redudant type from XmlParser method
Add Iterator trait for XmlLexer
Add test for str_read()

- Adds test to test what happens once reader goes into restricted char
  and/or end file. Expected behavior is to ignore them and peek anyway.
- Note change behavior in future to report RestrictedChars and EOF
Add method peak_str, extended Character enum with stringification

- Method peak_str is added. It attempts to read string and then return
  position to where it was. TODO test it
- Character enum now derives ToStr