Remove unused functions and imports
Add rewind for wrong characters
Changed documentation of Symbol <![
Remove Nmtoken implementation

- Remove Nmtoken parsing, which will be refactored at later point
Remove peek_chr and peek_str function from lexer

- These functions shouldn't be used, instead a combination of `read_chr`,
  `read_str` and `rewind` is recommended.
- Remove beforementioned functions and tests that used them
- Rewrite caller functions to use read_chr and read_str